Sedentary behavior in children (> 4 years of age) has increased over the last decades, resulting in a decline of physical skills. This leaves children prone to health risks such as obesity, mental depression, a lower self-esteem, and social exclusion. The risks increase particularly in children who develop a sedentary lifestyle. Exergames seem promising in engaging children in physical activities. Professionals from a variety of disciplines are thus interested in using exergaming in their work. Although there are promising results showing the potential effectiveness of exergames to increase the amount and quality of physical exercise, many questions remain to be answered. Chief among them is the question of which specific aspects of the exergames are responsible for the behaviour change process. The aim of our research project is thus to gain a greater understanding of the design space for exergames to support children in their physical behaviour. In our preliminary research we used a participative design method. Domain experts were invited to partake in interviews and co-creation sessions. These activities led to an in-depth list of requirements on a platform and exergame level and a first overview of necessary design directions. Future directions for this project include a participative design research study in which a platform containing multiple exergames will be developed and evaluated on a national scale, resulting in a knowledge base for dissemination in relevant fields of work.