BACKGROUND: Mask use is a critical precaution to prevent the transmission of SARS-CoV-2 in a crowded or densely populated indoor environment. There is still a lack of large-sample studies on mask use in children during the COVID-19 pandemic. METHODS: A questionnaire was distributed to individuals under 18 years of age from the pediatric outpatient department from November 2021 to May 2022. Participants who were willing to be interviewed and had good communication and judgment skills participated in our study. RESULTS: 5053 (a boy-to-girl ratio of 1.13:1 and a median age of 5 years) from 6200 individuals admitted to the pediatric outpatient department were enrolled in the study. The mask-wearing time increased in parallel with age. Children aged 3-5 years wore masks more correctly (χ = 41.591, P < 0.05), complained more about the discomfort (χ = 193.871, P < 0.05), and their parents/caregivers were significantly better aware of the preventive effect of masks on respiratory disease (χ = 19.501, P < 0.05) than parents/caregivers of other age groups. Masks designed for children were more used by those aged 3-5 years in outdoor settings. The commonest adverse events of mask-wearing were respiratory symptoms (61.2%), followed by dermatological symptoms (28.9%) and psychological symptoms (19.7%). Girls wore masks for a longer time and more correctly (χ = 10.598, P < 0.05) than boys. Compared with the pre-COVID-19 pandemic, wearing masks could significantly decrease the median frequency of respiratory infections during the COVID-19 pandemic (2[1-4] vs 3[2-4]; z = -2.692, P < 0.05). CONCLUSIONS: Wearing proper and well-fitted masks could significantly protect children from respiratory infections in a crowded or densely populated indoor environment during the COVID-19 pandemic. However, mask-associated adverse events, particularly in psychological symptoms, are needed to draw adequate attention, calling for early identifications and psychological interventions.