We explored children's physical activity level and psychosocial outcomes in a service-learning program. The program, taught by preservice physical educators, engages children in fun activities within a mastery motivational climate (MMC). Forty-two children in grades K-4 wore accelerometers during two 90-min sessions, resulting in 84 data points. Forty-three children age 8 and up completed a postprogram survey assessing motivational climate, perceived competence, and personal and social responsibility. Of those, 27 completed a presurvey as well. On average, children were engaged in light activity for 38.6% and moderate-to-vigorous activity for 26.0% of the program time. Social responsibility improved from pre- to postassessment. Children perceived a high MMC, and regression analyses showed that MMC was positively related to perceived competence, personal responsibility, and social responsibility at the end of the program. Results suggest this program was effective in using an MMC to promote social responsibility while engaging students in light, moderate, and vigorous physical activity.