Objectives: This study aimed to identify changes in the children’s happiness and in the paths from their academic performances, school adjustment after elementary school entrance, and the mediating effects of self-esteem and moderating effects of gender.Methods: Data from 400 children were used from the 8th to the 10th wave of the Panel Study on Korean Children. Children answered questions about their happiness and self-esteem, and their teachers rated academic performance and school adjustment. The latent growth model, path analysis, boot-strapping, and multi-group analysis were used with AMOS.Results: Children’s happiness showed the 2nd-year-change model. Gender was a significant predictor of their initial happiness values. Paths to children’s happiness were different based on grade. First-graders’ happiness was directly explained by school adjustment and indirectly explained by their academic performance through self-esteem. Second-graders’ happiness was affected only by their self-esteem; the school adjustment also showed significant direct effects on happiness in the third grade. The mediating effects of children’s self-esteem on academic performance and happiness were only evident in the first year of school. Finally, children’s gender didn’t have any significant Δχ² differences in the path of all the grades.Conclusion: Children’s happiness starts at different levels by gender and increase after the first year of elementary school. Children have different paths to happiness depending on the grade. Self-esteem fully mediates academic performance and children’s happiness in the first-year of school, and school adjustment directly affects children’s happiness. Paths to children’s happiness are similar between boys and girls.