While an extensive literature has shown that family structure is linked with child wellbeing, less well understood is how the dynamics within similar types of families affect children. Family systems theory posits that parents’ couple relationship is important for promoting children’s wellbeing. In this paper, we use data from the Fragile Families and Child Wellbeing Study to examine how couple supportiveness in married and cohabiting families is related to trajectories of children’s externalizing and internalizing behavior problems throughout early and middle childhood.