With rapid development of modern information and computer and network technology, the realization and application of digital library has infinite broad prospect. Children digital library is an online library that spreads digital information through web, storing and managing the content of multimedia literature information in digital mode and transmitting various information through online activities. As children have their own characters, we should have an in-depth study in this realm, providing valuable information for people who are contributed in this field and promoting better construction.
In this paper, we refer to more than one hundred papers related to this topic (from 1970 to 2010) and study from four aspects: â‘ children's information seeking behavior â‘¡information technology design methodology â‘¢information technology application â‘£ digital resources and online information service. In addition, we take ICDL as an example to analyze based on the four aspects we study. The main purpose of this paper is to review the children digital library and reflect the background, current research status and development trend of it.
2. Changes from Traditional to digital one
Â Â Â Modern technology leads the evolution of the traditional library to digital library. There are many factors to make the change. The demands for Information, limitations of resources available, searching difficulties in traditional libraries, low cost of using technology, the space needed to build a traditional library and needs of new generations are the most important points among these factors. For these changes, users can get the information on their own computer by using the Internet at anytime and anywhere. Moreover, all of the users can access the same resources with the most updated at the same time and in an organized way. Digital libraries use a very little physical space and the cost of maintaining it is considerably cheaper than that of a traditional one. So, the digital library becomes a popular one in coming age.
3. Literature Review
Nowadays digital library for children is not so popular. But advanced digital technology has potentially to persuade the children with color, animation, game, sound, interaction and other activities for the familiarity with digital media such as computer and IPhone. On the other side, books are important in children's learning period . In our observation, we study about the children who want to visit in digital library. Thus why, most of the parents are eager to be the trade-off between reading the books and using the computer in which the children's learning experience .
3.1. Children's information seeking behavior
Usually it is hard for children to express their ideas accurately, so they have difficulties when communicating with adults. Thus, adult's subjective feeling and evaluation becomes the main channel to understand children's information seeking behavior. However, a large number of experimental researches have shown that children's information seeking behaviors have specific characteristics. They are simpler than adult's i.e difference on age, knowledge structures and undeveloped cognitive skills.
3.1.1 Information reaction
The previous study shows that children have cognitive difficulties constructing effective search queries, and most of them cannot complete the task successfully . Compared with adults, children have a lower cognitive and thinking level . Thomas (1980) noted that it has a direct effect on information acquisition, because it could slow down the speed of identifying and evaluating information . The same results are shown in another experiment. In a multiple-part research project, which results from more than 70 experiments, Kail (1991) found that information processing speed increases exponentially with age . Furthermore, some researchers found that children lack of flexibility when using mouse to drag target [18-]. Therefore, many research teams focus on developing a visual interface and a simple and convenient operating system to achieve their tasks  .
3.1.2 Information retrieval and browsing
In the analysis of children's information seeking behavior, Bilal (2000) found that children did not search efficiently or effectively due to individual differences . They are interactive information seekers, preferring to keyword searching and browsing approaches, for example they like to browse hyperlinks, rarely use advanced search features, and backtracked multiple times to revisit previous pages . Further, when using keywords retrieval, they have difficulty in inputting correct spelling due to limited vocabulary, and generally they like to use natural languages and phrases. For the retrieval categories, less hierarchy and wide scope meaning ones are in accordance with children's cognitive features .Comes to the operation; Revelle et al. (2002) studied QueryKids as an example found that children obtained good retrieval effectiveness when they construct both simple and Boolean queries .
3.1.3 References selection
Children's interest in reading differs with age. They choose books primarily by cognitive feeling. The book selection criteria for children ages 4-7 are how the book looks, as well as how the book makes them feel. That means children ages 4-7 mainly based on the physical feature of literatures such as color for cover, picture, shape, size etc, to select a book. They are fond of reading interesting books like novels and fairy tales. Children ages 8-11 are able to choose books primarily by content abstract in cover page or title page. They prefer books about science and animals or the content have certain scientific and knowledge .
Information technology design methodology
The good design features and children needs are important role in consideration. Designs must be different from adult with their culture, norms and complexity and moreover they still require for spelling, reading and other ability  . At the early stage (2 to 7), children are generally learning period to know the vocabularies and may have no knowledge of alphabet yet . Thus why, audio, graphics or animation should substitute for all the written functionalities in a computer system  . Also, buttons using symbols and animations are more convenience than textual forms in which the interface of digital library for children . These alternatives enable children to understand the application and to aid them in using the Interface easily. As the Durin research, the icons of digital library for children should be graphical representation and animation . The children are inconvenient for concentrating on small buttons compare to adult . Because of it, the User Interface for children should be large and supportive of surfing in order to avoid using alphabets.
Children are more eager to share their idea with their partner . Thus why, collaboration play major role in children's learning system. It means that a person shares the knowledge, experiences and skills in order to others and gain new feedback in return . The task of planning and organizing collaboration enhances the opportunities in the education of children . The digital library for children should provide the opportunities to create the own stories and other children can comment on it. They can share their knowledge and experience in it as a collaboration environment. As Y.L. Theng said that "In order to be popular the digital library for children and having fun, they are also allowed to be both readers and writers" .
Moreover, digital library for children should support them opportunity to rate the book, give comment, write shot review and discuss their ideas .So, they can learn from each other like community area.
The search system for the children is the important point in usability issues. The browsing content interface should support as zoo area with animals to be familiar with children  Allison Druin exposed that design should support the opportunity that offer children multiple paths to find what they are looking for . The catalogue in the digital library for children should support more flexible than traditional one. To be international one, the design should support multi-language 
3.3 Information technology application
Children use information technology for various purposes to gratify their requirements. They can use information electronically anywhere and anytime. As digital information is ubiquitous and easily available, children can communicate environment around them by using technology and multi-media devices. Digital environments support children to be master of knowledge for creativeness, decision making and problem solving. To be an appropriate digital environment for children requires design and content choices that are concerned with developing needs of children .
Children are more exposing to the digital environment rather than exposing to the books. They utilize digital tools and electronic information in their daily activities. Using these facilities sometimes may be too expensive or difficult because of updating hardware, software and contents. Digital Libraries provides contents to students with free of charge in order to get access to the information resources directly without taking into account distance, time or financial support .
Digital library is becoming a place for children to support educational learning objectives. The digital library tools must be proper with children for home and collaborative use . Technology could be a link to using digital library. Technology for digital library supports children to be easy in searching, browsing, accessing and reading books. The response time and processing time in digital library should be fast because children want to access information as fast as possible. If speed is not fast enough, they will be disappointed and will not use this library again. Technological specifications such as operating systems, file formats must be considered in creating a digital library. The digital library should be accessible by various operating systems, for example, Windows, Linux and Mac Os.  When digital library share the books to children all over the world, they should follow copyright policy. Library should not allow downloading, printing of books without authorization of rights holders. If the users want to download or print the book, they should request to the rights holders to obtain permission. 
3.4 Digital resources and online information service for children
Â Â Â Â Â Â Â Â The children use many new technologies in recent years and engage in a lot of digital resources. These digital resources come from various aspects, such as CD-ROMs, drawing tools, computer games, mobile phones, the Internet, PlayStation games and texting . Children are facing so many hi-techs, and then the print version information becomes auxiliary source because digital texts contain much richer and up-to-date information . Based on this, digital resources have a big effect on children. Since children in different age level will have their own definition of accepted culture and have their own interests and preference , and children usually use different criteria, for example, the number of illustrations, preferred genres and recommendations, to select resources , the selection should be based on in-depth study of children's psychological characteristics, reading interest and patterns. In other words, the collection and selection of children digital resources should be diverse, not only reflect resources from different national culture, but also balance the proportion of serious resources and interesting resources, such as novels and myth stories. Privacy on the Internet is another issue that needs to be concerned, because digital resources provide many ways and opportunities for children to obtain various information. The world of cyberspace is an attractive and seductive environment for children , and are often have limited self-regulation to resist these. As a result of it, the U.S. Federal Trade Commission makes effort to deal with this problem. The laws have the privacy policies and set legislation to control the collection of information provided for children on the Internet . Some measures should be taken in school and public libraries to protect the children, avoiding contacting potentially harmful digital information, such as member service. In digital library, separate index is created for each language and users should select a language when searching
Â Â Â In information age, the Internet, library websites and digital library provide increasingly rich digital resources for children's learning and life entertainment. For example, a large number of teachers select and use diverse online resources in the classroom to help teaching, and the number of these resources is increasing . Teachers can explain the knowledge with vivid images and news stories to their students to save much time in classroom. Computer game is another popular entertainment among children. These indicate that online information is becoming more and more popular and the service provided in various forms, such as online bookshop, network navigation, funny entertainment platform and etc.
Â Â Â Therefore, children digital library as an online service is very important among youngsters which provide rich resources and provide quick and convenient services. The construction of it can be paid more attention in the following days.
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We picked up ICDL to make analysis for our research. ICDL is one of the huge digital libraries for children and they made many researches to found this one. They explored a collection of more than 10000 books with at least 100 languages at the first step of their journey. Their main target users are children, teachers, parents and librarians. The following points are our analysis part.
The ICDL collaborate with children as design partners to develop interfaces and other functions so that a matured and full-featured system is available. The point-and-click and visual interface is exactly what children want to get. Moreover, the color of ICDL is proper and all operations are easy to handle. But vivid images and animations which will help children read and learn efficiently and effectively, are not enough.
There are four research tools to access and retrieve books. By selecting the simple search interface (figure 1), users can search for books with colorful buttons arranging in various categories. The advanced search (figure 2) enables user to search books in a text-link-based interface. The location search surface (figure 3) is an interesting place where children may spin a global and select a region to browse. For the keyword searching interface (figure 4), children should type in keywords with natural language.
The ICDL is a collection of ten thousand children's books which provides many options to children for searching, browsing, reading, and sharing books. These popular categories were chosen based on research with children and librarians concerning how children want to look for books . As a result, children will choose their favorite according to their own willing. The ICDL has distinguished functions and significance which is different with adult digital library. It meets children's seeking needs and behaviors and supports them to find and read books with highly visual interface and a simple and convenient operating system. ICDL use large buttons to understand for children instead of with textual form. Having fun for children, it use the graphical representation (figure 1) in the search area. Moreover, the interface design use four section in search like map (figure 1).Â ICDL software can be accessible in various operating systems such as Windows, Solaris, Linux and Mac OS. So children can access this library by using different operating systems. ICDL value the rights of intellectual property and they examine the books carefully before putting into their library. If the books that have not rights holders, these books can be put in the ICDL library with no permission from any person. ICDL requests to the owners or creators to get permission when they want to publish new books. One fourth of the books in ICDL are within the copyright. Â The speed, response and process time of ICDL are fast enough so as to browse, search or read information.
Â The ICDL collects more than 10,000 books in at least 100 languages that represent outstanding historical and contemporary books throughout the world, so children can obtain information they need conveniently and appreciate the riches of children's literature. It provides easy access for users to get the information, including many search ways. It also provides much online service, such as member service, community forum, and so on. It is a very helpful online library.
5. Conclusion and further words
Children are very important in the future. It is said that "youth of today are leaders of tomorrow". Children are dominated from manners and attitudes that are occurred in their society. Digital library supports children to be more valuable person. The prospects of children for their exposure with books grow as the nature of their particular experiences. Digital libraries become useful among children because they are fun and comprehensible and allow children to be both readers and writers. The attitudes of children may develop if they have chances to share their interests, experiences and stories in the library. Therefore, digital libraries are becoming places to support educational learning objectives for children.