Abstract Primary Subject area Public Health and Preventive Medicine Background Various studies have examined the general impact of the COVID-19 outbreak on children’s health behaviours. The impact of public health measures practised by children during COVID-19 is relatively unknown. Objectives The primary objective was to determine the association between physical and social distancing measures and children’s outdoor time, sleep duration, and screen time during COVID-19. Design/Methods A longitudinal study using repeated measures of exposures and outcomes was conducted in healthy children (0 to 10 years) between April 14 and July 15, 2020. Parents were asked to complete questionnaires about isolation, physical distancing practices, and children’s health behaviours. The primary exposure was the average number of days that children practised physical and social distancing measures per week. The three outcomes were children’s outdoor time, total screen time, and sleep duration during COVID-19. Linear mixed effects models were fitted using repeated measures of primary exposure and outcomes. Results This study included 554 observations from 265 children. Physical and social distancing measures were associated with shorter outdoor time (-17.2; 95% CI -22.07, -12.40; p < 0.001) and longer total screen time (11.3; 95% CI 3.88, 18.79; p = 0.003) during COVID-19. The association with outdoor time was stronger in younger children (< 5 years), and the associations with total screen time were stronger in females and in older children (≥ 5 years). Conclusion Physical and social distancing measures during COVID-19 have resulted in negative impacts on the health behaviours of Canadian children living in a large metropolitan area.