Many children do not realize the physical health, mental health, cognitive, and academic benefits of physical activity because they are insufficiently active. Effectively promoting physical activity in children requires understanding the determinants of physical activity. Parent physical activity and support for physical activity have emerged as central influences, but few studies have applied longitudinal designs and device-based measures of child physical activity. The purposes of this cohort study were to examine direct associations between parent physical activity and child physical activity, parent physical activity and parent support for physical activity, and parent support and child physical activity; and to examine indirect associations between parent physical activity and child physical activity, mediated through parent support behaviors. We used structural equation modeling with panel analysis to examine direct and indirect influences of parent physical activity and support on 7th grade children’s physical activity, adjusted for 5th grade levels. Parent surveys were administered at the 5th grade time-point. Parent physical activity indirectly affected child physical activity, mediated through the parent support behavior of providing transportation for physical activity. Parent physical activity was also directly related to four parent support behaviors. Increasing parent support for child physical activity, and possibly parent physical activity, may be effective approaches to increasing child physical activity.