The promotion of physical activity (PA) in children and their parents requires effective planning and sometimes even interventions. This study shows the effect of PA during a 15-week intervention program "Junior for Seniors" by applying a socioecological model to the interpretation of the data. This comprehensive approach emphasizes the fact that health promotion should focus not only on intrapersonal factors but also on the multilevel factors that might be determinants and modulators of increased PA. In 2015, 24 children ("juniors," 14 girls and 10 boys, aged M = 7.96 ± 0.69) and 22 parents ("seniors," 14 mothers aged M = 38.86 ± 2.96 and 8 fathers aged M = 37.38 ± 2.97) were voluntarily enrolled in a study spread across three primary schools in the city of Poznań, Poland. The effectiveness of the intervention was determined according to postintervention behavioural changes in PA in comparison to preintervention levels, as reported by the parents and children. Overall, the study found increases in PA levels and reductions in sedentary time. Although the changes are modest, there are some unrecognized benefits of the intervention which may have occurred, such as improved sport and motor skills, more frequent family social behaviours (walks, meals, and visiting relatives), or simply improved quality of "do-together" leisure time PA.