This study investigated whether maternal perceptions of child body mass status would predict child body mass index (BMI) z-score via two sets of sequential mediators: (1) four maternal practices promoting child energy expenditure and (2) children’s energy expenditure behaviors. The data of N = 729 mother–child dyads were collected at baseline [T1; n = 495 at 7- to 8-month follow-up (T2)]. Mothers reported perceptions of child body mass status and maternal practices (T1); children reported sedentary screen use and physical activity (T1, T2). Child body mass was assessed objectively (T1, T2). Higher stimulation to be active (T1) was related to a lower child BMI z-score (T2) via higher levels of child physical activity (T2). Higher levels of monitoring of screen use (T1) were associated with higher child BMI z-score (T2) via lower levels of child physical activity (T2). Encouraging parents to stimulate their children to be active may be beneficial for children’s weight maintenance.