In recent years, children have begun using computers actively. With this situation, they are beginning to learn how to operate a mail user agent (Hereafter, called MUA) to adaptation to an information society. However, the user interface of general MUAs is not designed with them in mind. Specifically, many unnecessary menu items exist on the user interface of general MUAs. Moreover, the menu text contains many difficult characters (called Kanji) in Japan. In this situation, it is difficult to say that children could learn how to operate the MUA without being confused by the complex user interface. We consider it to be important that even children who have not learned Kanji can use the user interface. To solve this issue, we have developed a MUA that has a simple user interface only represented in Hiragana. In order to create a simple user interface for child use, it is necessary to reduce the number of menu items. Along with this, the number of available functions on this MUA has decreased. However, we think it is rather important to prevent the confusion and unexpected behavior of children. It is also necessary to devise a function to make an e-mail to attract the interest of them. Therefore, we have implemented a function to generate pictograph mail easily. We are using the XML-based User Interface Language (Hereafter, called XUL) to build a user interface for children. It is a language used to build the user interface of Mozilla applications like Firefox and Thunderbird. By using it, we can customize the user interface of Thunderbird to fit any purpose. By these improvements, we think children will be able to use the MUA actively and with enjoyment without falling into confusion.