Children and adolescents spend considerable time online. The current study investigated moderators of short-term longitudinal associations from social media use to changes in body satisfaction and physical activity. Participants (144 girls, 152 boys) were third-sixth grade students (ages 8-13) attending public schools in Florida (USA). Self-reports of social media use, body satisfaction, physical activity, and susceptibility to peer influence were completed twice, approximately 13.5 weeks apart. The results indicated that higher initial social media use was associated with greater subsequent decreases in body satisfaction and physical activity, but only for children high on susceptibility to peer influence. The findings suggest that peer conformity amplifies the risks of social media use. The fact that meaningful change can be identified across a relatively short period of time is noteworthy and demonstrates the need for careful monitoring of those who participate in social media at an early age.