The design goals of recently developed serious games are to improve attention, affective recognition, and social interactions among individuals with autism. However, most previous studies on serious games used behavioral questionnaires to evaluate their effectiveness. The cognitive assessment of individuals with autism after behavioral intervention or drug treatment has become important because it provides promising biomarkers to assess improvement after cognitive intervention. In this study, we developed a game-based social interaction platform incorporating an eye-tracking system for children and preadolescents with autism. Three modules (focusing on gaze following, facial emotion recognition, and social interaction skills) are included in the platform; participants with autism learn these according to their cognitive abilities. The eye-tracking results showed decreased fixation durations when autistic children looked at positive emotional expressions and focused on multiple targets. Prolonged saccade durations and shorter fixation times for social-related facial emotion expressions were also found in preadolescents and teenagers with autism. Our findings suggest that these atypical gaze patterns are reliable biomarkers for evaluating the social and cognitive functions of autistic individuals while playing serious games. The proposed platform's game-based modules and the findings regarding aberrant gaze patterns in autistic individuals demonstrate the possibility of evaluating cognitive functions and intervention effectiveness by using eye-tracking signals in a serious game or real-life environment.