Research Findings: The relationship between teachers and children in kindergarten is vital for children’s social development. Moreover, children’s characteristics have an impact on this relationship. The present study utilized data from 54 teachers and 566 kindergarten children in Finland with the aim of investigating the cross-lagged associations between teacher–child relationships and children’s social competence. Additionally, this study explored the moderation effect of children’s achievement strategies on the dynamics between the teacher–child relationship and children’s social competence. The results of cross-lagged panel analyses revealed reciprocal associations of conflict with both prosocial and antisocial behaviors and a positive predictive association of closeness to prosocial behavior. Moreover, through multigroup analyses, we found that children’s passive-avoidance behavior had a moderating effect on the positive predictive association of prosocial behavior to closeness. Our findings underline the important role of teachers in building relationships with children, while also recognizing children’s impact on kindergarten social dynamics. Practice or Policy: The results highlight policy and practice that encourage kindergarten teachers to support children’s social competences through building warm and supportive relationships with each child.