This study estimates a production function of human capital at the early stage using various classes of inputs, including prior child's skills, parental material and time investments, household wealth, and an early childhood education intervention using data from the RIECE Thailand project. The first part of this study focuses only on the short-term impact of an early childhood education intervention. The results, based on an instrumental variable approach, show that an early childhood education intervention in rural Thailand has a significant short-term impact on child development, including gross motor, fine motor, expressive language, and personal and social skills. A reduced-form estimation using the randomization of additional teachers as a key explanatory variable, gives similar but smaller estimates relative to the one that uses the randomization as an instrument. The results are robust to various estimation methods, child development measures, and sample selections. In addition, we found that the impact of the new curriculum is quite homogeneous across sub-groups except in some dimensions, notably parental absence, main caregiver's working memory, mother's age, and teacher's job status. The second part extends the first one by estimating a production function of child development with more classes of inputs, including prior child's skills, parental material and time investments, and household wealth at the baseline. The results indicate that the new curriculum still has a significant impact on child development in all five domains while an increase of material and time investment boosts child development on expressive language domain only. The stock of child's skill at the baseline and household wealth have no significant impact on later developmental outcomes of the child. The results are robust to various estimation methods, child development measures, and sample selections.