This study examined the moderating role of child negative affect in the link between coparenting cooperation and child social competence. Cross-sectional data were collected from a community sample of 258 Chinese families with preschool children (mean age = 5.19 years; 47% of them were girls), residing in Hong Kong, China. Using questionnaires, parents (80% of them were mothers) rated their children’s negative affect as well as their own coparenting cooperation, dyadic, parent-child nurturance, and marital love. Also, class teachers rated children’s peer acceptance and social cognition using questionnaires, and children reported their own social competence in structured interviews. Multilevel and multiple regression models indicated that, controlling for dyadic, parent-child nurturance, and marital love, coparenting cooperation was positively associated with teachers’ ratings of child peer acceptance and social cognition and children’s reports of their own social competence. These associations, however, were significant only for children with high negative affect. Theoretically, these findings highlighted the importance of considering child dispositions when examining the contributions of family relationships to child development. Practically, parents, especially those with temperamentally difficult children, should learn to work cooperatively with each other to promote their children’s social competence.