BackgroundCo-participation in physical activity may be a useful strategy for increasing physical activity in mothers and their young children, yet little empirical evidence exists on this topic for young families. This study aimed to identify the prevalence of mother-child co-participation in physical activity and examine the association between co-participatory behaviours and objectively-assessed physical activity in young children and their mothers.MethodsOne-hundred twenty-three 4–6 year-old children and their mothers were recruited from preschools in Belgium between November 2010 and January 2011. Mothers completed a questionnaire assessing the frequency of co-participation in five activities. Both mothers and children wore ActiGraph GT1M accelerometers concurrently for 7 days to assess the time spent in moderate-to vigorous-intensity physical activity (MVPA) and light- to vigorous-intensity physical activity (LMVPA). Descriptive statistics (means, frequencies) were used to determine the prevalence of co-participation. Separate multiple linear regression analyses examined the association between co-participation and mothers’ and children’s physical activity on weekdays and weekends.ResultsMost mothers reported infrequent co-participation in physical activities with their children. On weekdays, walking or cycling for short trips was positively associated with children’s MVPA while attending a park or similar more than once per week was negatively associated with children’s MVPA and LMVPA. Going to an indoor play centre together once or more per week was negatively associated with mother’s LMVPA. On weekends, walking or cycling with their child in their free time was positively associated with both children’s and mothers’ MVPA and childrens’ LMVPA. Going to an indoor play centre together 1–3 times/month was negatively associated with children’s weekend MVPA.ConclusionsReported rates of co-participation in mothers and their preschool children were low. The association with maternal and child physical activity may be dependent on the co-participatory behaviour assessed and may differ between weekday and weekends. Promoting walking and cycling together during leisure time may be an effective strategy to increase both preschool children’s and mothers’ MVPA.