Objective: This study aimed to explore the effect of the national preschool curriculum on children’s development in Korea, focusing on teachers’ daily practice. By comparing the effect of the teachers’ curriculum practice to that of quantitatively measured free play, it tried to add practical implications beyond the statistical significance. Methods: Participants were 512 three-year-old children who participated in the Panel Study of Korean Children and their teachers. National curriculum practice and free play time at the age of three was put in the hierarchical linear regression models to discover children’s developmental outcomes at the age of four, in domains of language, cognitive development, and social development. Results: Results demonstrated the significant positive influence of national curriculum practice on every domain of developmental outcomes. However, no facilitative influence of free play time was observed. Conclusion/Implications: The importance of teachers’ practice of the national curriculum was emphasized. It was implied that the quantity of free play time itself did not assure the sound development of children. Policy implications were discussed regarding teacher practice and education.