Summary 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.