This study aimed at examining reciprocal associations among teacher–child interactions, teachers' work engagement, and children's social competence across grade 1. The participants were 51 Finnish teachers and 815 children. The quality of teacher–child interactions (emotional support, classroom organization, and instructional support) was assessed with the Classroom Assessment Scoring System. In addition, teachers reported their work engagement and rated children's social competence (prosocial and antisocial behaviors). The results of the multilevel modeling indicated that high-quality instructional support was associated with more prosocial and less antisocial behavior. Prosocial behavior was associated with higher-quality instructional support. Work engagement was associated with less antisocial behavior, and prosocial behavior with higher work engagement. Finally, work engagement was associated with higher-quality teacher–child interactions. The results suggest that teachers can enhance children's social competence with high-quality instructional support. Moreover, children's prosocial behavior can be reflected in teacher–child interactions and in teachers' work engagement.