Background Measures during the COVID-19 pandemic, including the closure of schools and sports facilities, may have lasting impact on the physical activity (PA) of children that persists for a long time.Objective To investigate the effect of COVID-19 measures on screen time and PA in Dutch children pre-, during- and post-school closures.Methods In cohort A (n = 102, 10.5 +/- 3.6 years, 42.4% boys), data on PA and screen time during the lockdown were collected using a questionnaire. In cohort B (n = 131, 10.2 +/- 0.9 years, 43.5% boys), data on PA and screen time were collected using a questionnaire and accelerometry 1 year before and after school closure.Results In cohort A, 62% reported less total PA. Self-reported screen time on week days increased 34 +/- 105 min/d during the lockdown. In cohort B, sedentary time as measured by accelerometry, increased by 45 +/- 67 min/d and only 20% reached PA levels of 60 min/d compared to 64% in May 2019. Self-reported screen time increased by 59 +/- 112 min/d and 62 +/- 130 min/d during week and weekend days, respectively.Conclusions Children were less physically active, and screen time was higher during and after the school closures due to the COVID-19 lockdown. This is alarming as an active lifestyle in children is crucial in preventing chronic diseases such as obesity.