For the Girls' Day web application at Freie Universität Berlin the Human-Centered Computing (HCC) Research Group developed a concept for an appropriate user interaction design, the information design as well as the user interface design. The web application offers girls at the age of 10 till 15 years possibilities to register for the Girls' Day workshops at the FU Berlin, get information and manage their profile information. The web application requires to be easy to use for even the younger girls. Our interaction design, information design and interface design therefore strictly follows usability criteria and important design issues for kids. The software is also used as a workshop management system for the university lecturers and the organization team. Lecturers and team members use the software to generate workshops, publish them and administer participants. The HCC Research Group leads the whole Girls' Day software project and coordinates the implementation of the web application for the girls as well as the workshop management system in an agile process.