Research has shown that many aspects of family functioning are directly and indirectly linked to children’s social development. One important aspect of how families function is family structure. In this regard, families have undergone tremendous change over the last decades resulting in increased cohabitation and divorce. These types of families are believed to be more unstable than married families. Instability creates more stress that can be difficult for children to cope with resulting in increased behavioral problems. Although past studies have shown an association between union instability and children’s externalizing problem behaviors (EPB), the mechanism by which this occurs is less understood. Using Family Systems Theory and data from the Fragile Families and Child Wellbeing study (n = 3,387), I examined whether family processes – father and mother involvement, co-parenting support, and maternal responsiveness – explained the association between union instability and children’s EPB at 9 years. I also examined whether marital status and children’s temperament moderated this association. Using measured variable path analysis, I found that only co-parenting support mediated the association between union instability and child EPB, and only for children whose mothers experienced a divorce (not a nonmarital separation), controlling for known covariates of children’s EPB. The association between union instability and children’s EPB through co-parenting and parenting was not moderated by child temperament. These findings suggest that co-parenting rather than parenting explains children’s social adjustment when families undergo a divorce.