Background In many countries, the restrictions related to the first period of lockdown during the coronavirus disease 2019 (COVID-19) pandemic led to widespread changes in health service usage in general and in emergency departments in particular. However, no comprehensive evaluation of changes has been published to date. The objective of the present study was to determine the precise impact of the 2020 lockdown on admissions to a pediatric emergency department (PED) compared to the same periods in 2018 and 2019. Methods This retrospective, observational study included all patients under the age of 183 months (15.25 years) admitted to our French university hospital's PED during the period from March 17 to May 11 in the years 2018, 2019, and 2020. The primary outcome was the change in PED admissions in 2020 compared to 2018 and 2019. The secondary outcomes were notably changes in the primary discharge diagnoses, the discharge destination, and unwarranted visits. Results A total of 10,479 PED visits were identified, of which 10,295 were analyzed. In 2020, the number of PED visits fell by 61% and 63% vs. 2018 and 2019, respectively. Although the number of discharges to other hospital departments decreased by 52% and 49%, the proportion of these discharges increased: 18% of 1579 in 2020 vs. 13% of 4232 in 2018 and of 4484 in 2019 (p<0.01). Discharge from the PED to the intensive care unit was significantly more frequent in 2020 (p<0.05). Unwarranted visits were significantly lower in 2020 (19%) as compared to 2018 (22%) and 2019 (24%). Surgical and injury-related discharge diagnoses increased by 6% in 2020 (p<0.001), with a significant rise in trauma and foreign-body injuries (p<0.05). With regard to disease-related discharge diagnoses, we observed a significant rise in mental, behavioral, and social issues (p<0.01). Conversely, there was a significant (p<0.01) drop in diagnoses of acute infectious diseases in 2020 compared with 2018 and 2019. Conclusion Lockdown was associated with a massive reduction in the number of PED visits, a significant change in primary discharge diagnoses, and a decrease in the proportion of unwarranted PED visits compared to the previous 2 years. This should encourage public health researchers to examine how to alleviate the burden of unnecessary PED visits.