This study aims to describe the prototype development and testing of a serious game designed for Brazilian children with diabetes. Following an approach of user-centered design, the researchers assessed game's preferences and diabetes learning needs to develop a Paper Prototype. The gameplay strategies included diabetes pathophysiology, self-care tasks, glycemic management, and food group learning. Diabetes and technology experts (n = 12) tested the prototype during audio-recorded sessions. Next, they answered a survey to evaluate the content, organization, presentation, and educational game aspects. The prototype showed a high content validity ratio (0.80), with three items not achieving the critical values (0.66). Experts recommended improving the game content and food illustrations. This evaluation contributed to the medium-fidelity prototype version, which after testing with diabetes experts (n = 12) achieved high content validity values (0.88). One item did not meet the critical values. Experts suggested increasing the options of outdoor activities and meals. Researchers also observed and video-recorded children with diabetes (n = 5) playing the game with satisfactory interaction. They considered the game enjoyable. The interdisciplinary team plays an important role guiding the designers in the use of theories and real needs of children. Prototypes are a low-cost usability and a successful method for evaluating games.