In this paper, we describe the roles played as well as contributions made by child participants in the design of an innovative communication tool for children with cancer. SISOM is a handheld, portable computer application with a graphical user interface that is meant to: (1) help children with cancer communicate their symptoms / problems in a child-friendly, age-adjusted manner; and (2) assist clinicians in addressing children's experienced symptoms and problems in patient care. Unlike other applications for children, the purpose of SISOM is not to provide information to ill children but to elicit personal information from them. Thus the application has a unique set of design issues. Healthy and ill children played an important role in different stages in the design process. They made significant contributions to the graphical design of the system's interface; selection of understandable, child-friendly terms used by the system to describe symptoms; iconic and graphical representations; and its usability. We describe the participatory design methods we used that included children and share important insights from this collaborative design process.