BACKGROUND: The Child Care Facilities Program (PEI, for its Spanish acronym) in Mexico targets parents of children aged 1-3 years living in poverty, providing education and care strategies 5 days a week. This study aimed to evaluate the impact of a 6-month stay in childcare centers on the developmental levels of children under 4 years old. METHODS: A longitudinal, before-and-after study was conducted. Children aged 12-42 months enrolled in the PEI were included in the study. All participants remained in the program for 6 months from the first measurement. The overall and area-specific developmental levels were assessed using the Early Childhood Development Assessment (EDI, for its Spanish acronym) test. The percentages of developmental levels (normal, delayed, and at risk of delay) were compared between the baseline and 6-month assessments using the McNemar test. RESULTS: The study included 1835 children, of whom 52% were male. The age distribution was as follows: 28.1% were 12-24 months old, 48.4% were 25-36 months old, and 23.5% were 37-42 months old. At baseline, 80.5% (n = 1,476) of the children were classified as having normal overall development, 16% had developmental delay, and 3.5% were at risk for delay. After 6 months, the percentage of children with normal development increased to 90.1%, while those with developmental delay and those at risk for delay decreased to 8.7% and 1.2%, respectively. Similar improvements were observed across various developmental areas, except in the knowledge area. CONCLUSIONS: A 6-month stay in childcare centers is beneficial for improving the developmental levels of children under 4 years old, both overall and in motor, language, social, and cognitive areas.