Summary Study aim: Both adequate time and quality time are important for the wellbeing of a family. The study evaluated the effective­ness of a 15-week family-based physical activity (PA) intervention entitled “Junior for Seniors”. It included both children and parents. The paper focuses on an important topic - family-based leisure-time physical activity (LTPA). It is set in Polish primary schools, which is a novel approach. Regular physical activity for young people and their parents improves health and fitness including strength and endurance, promotes development of healthy bones and muscles, helps control weight and in­creases self-esteem. Material and methods: In 2015, 24 children (“juniors”, 14 girls and 10 boys, aged (mean ± SD) 7.96 ± 0.69 years) and 22 par­ents (“seniors”, 14 mothers aged 38.86 ± 2.96 years and 8 fathers aged 37.38 ± 2.97 years) were voluntarily enrolled in a study encompassing three primary schools in the city of Poznań, Poland. To measure factors associated with LTPA, selected ques­tions from the Health Behaviour in School-aged Children (HBSC) questionnaire were used. Results: As a result of the intervention, children increased both the amount of time they spent on intense LTPA and the fre­quency with which they undertook LTPA (values for “time spent on intense physical exercises during leisure time” were statis­tically significantly higher in the post-test; Z = -2.57; p < 0.05). In the post-test, statistically significant relations were obtained between the following variables: reason for physical activity: for better sport results - with family: playing together; r = 0.41, p < 0.05. In the parents’ group no statistically significant increase was observed in any of the variables measured. Conclusions: The research issues presented in this paper require further exploration in larger scale studies. However, based on the present study, it may be concluded that one important goal has been achieved: that of more frequent family social behav­iours and improved quality of LTPA.