Physical activity levels among early adolescents and their parents are insufficient for health benefits. Identifying modifiable determinants in parent-child dyads can inform future research and interventions. We tested a partial least squares path model based on the integrated behavior change model in insufficiently active Finnish parent-child dyads (n = 88), including measures of autonomous and controlled motivation, social cognition constructs (attitude, subjective norm, perceived behavioral control), intention, planning, and habits. Autonomous motivation predicted attitude in both samples, but only predicted subjective norms and perceived behavioral control in children. Attitude in turn predicted intention, planning, and habit, in the child sample, but only intention and planning in parents. Perceived behavioral control predicted intention and planning only in children, while subjective norm had minimal effects in either sample. Autonomous motivation and attitude consistently determined intention and planning for both parents and children, highlighting their importance in future research and interventions.