Background Physical activity and diet are major modifiable health behaviors contributing to obesity risk. Although patterns of these behaviors tend to cluster within individuals and within family units, it is unknown to what extent healthy and unhealthy dietary intake might differentially accompany sedentary and physical activities in mothers compared with their children. Objective Our goal was to examine differences in co-occurrence of activities and dietary intake between mothers and children, as measured in real time using ecological momentary assessment.Participants/setting This study examined cross-sectional data from 175 mothers and their children aged 8 to 12 years. Main outcome measures Participants completed 8 days of ecological momentary assessment surveys, reporting on whether the following activities had occurred during the past 2 hours: sedentary screen activity, physical activity, and intake of healthy (ie, fruits and vegetables) and unhealthy (ie, fast food, chips/fries, pastries/sweets, and soda/energy drinks) foods. Statistical analyses performed Multilevel logistic regression models estimated the adjusted odds of consuming healthy and unhealthy dietary intake for mothers and children during time periods reporting physical activity (vs no physical activity) or sedentary screen activity (vs no sedentary screen activity). Post hoc tests compared estimates for mothers vs children. Results Children were significantly more likely than their mothers to consume unhealthy foods during 2-hour windows that included physical activity (odds ratio [children] 1.85, 95% CI 1.47 to 2.31; odds ratio [mothers] 0.83, 95% CI 0.58 to 1.20; Pdiff <0.05), but not sedentary screen activity (Pdiff=0.067). In addition, children and their mothers did not differ in their likelihood of consuming healthy foods during 2-hour windows with sedentary screen activity (Pdiff =0.497) or physical activity (Pdiff =0.170). Conclusions Results indicate that the consumption of unhealthy foods may be more likely to co-occur within a 2-hour window including physical activity in children as compared to their mothers. Future research should examine reasons for this difference, and potential areas for intervention.