Introduction - In France, the Departmental Maternal and Child Health Service is responsible for organizing health screenings in preschool (BSEM) for 4-year-olds. The objective of this study was to examine, over three school years (2018–2019, 2019–2020, and 2020–2021), changes in children’s weight and height status and to estimate the impact of the SARS-CoV-2 health crisis in Val-de-Marne. Materials and Methods - From the data provided by the BSEM, we extracted the following information: weight, height, sex, age, whether the school was located in a priority area, and whether the child attended a school cafeteria or daycare. We calculated the Z-score of the body mass index (BMI-Z) using the LMS method. Results - Among the 48,119 children analyzed, the proportion of children who were overweight according to BMI-Z was significantly higher in 2020–2021 compared to previous years. In multivariate analysis, this increase in overweight and obesity was confirmed for the 2020–2021 school year compared to 2018–2019. The identified risk factors are female gender and the school’s affiliation with a priority or reinforced priority education network. Attending daycare or the school cafeteria are protective factors against overweight and obesity. Discussion and Conclusion - BSEM data show that the increase in the body mass index of children in the middle year of preschool is significant following the health crisis. The measures taken have increased sedentary behavior and worsened dietary habits, with a significant impact on children. Particular attention must be paid to socially disadvantaged areas and to girls.