Purpose: Physical inactivity and overweight status has been linked to low socioeconomic status (SES) in youth. Parents are known to influence both their child’s weight and physical activity (PA). The relationship between parent and child PA is of interest to many researchers; however, previous research typically relies on self-reported measures. The purpose of this study was to examine the relationship between parent and child moderate-to-vigorous PA (MVPA) and body mass index (BMI) in a sample of children (4–11 years old) using wrist-worn accelerometers and to explore mediating processes by which SES influences child MVPA and BMI through their parents MVPA and BMI. Methods: Parent and child dyads (n = 174) wore an ActiGraph GT3X+ accelerometer on their non-dominant wrist for 7 days. Mediation analyses were conducted to understand the indirect relationships between SES and child MVPA and BMI. Results: Weekend parent and child MVPA was significantly related (p < .01). Parent and child BMIs were also significantly related (p < .001). There was a significant negative direct effect of SES on child BMI (p < .05). Additionally, we observed a significant negative indirect effect of SES on child BMI via their parents BMI (B = −.04, SE.02, 95% CI = −.07 to −.01). Conclusions: Whilst parent and child MVPA were significantly related during the weekend, there were no associations between SES and MVPA. Future interventions aiming to improve health outcomes in children should consider the influence SES can have as well as parental activity on children’s weekend MVPA.