The nature of parents’ romantic relationships has been shown to relate to how parents interact with their children and how their children develop. This study used a subsample of 1,318 couples from the Fragile Families and Child Wellbeing Study across 8 years to test Belsky’s model of the determinants of parenting. A dyadic path analysis was used to test the effects of relationship quality on negative child behavior, mediated by positive parental engagement. Results revealed that mothers’ and fathers’ early relationship quality predicted their own parental engagement 2 years into the future. Additionally, mothers’, but not fathers’, positive parental engagement predicted negative child behavior. Last, mothers’ parenting behaviors mediated the relationship between relationship quality and child outcome. The results lend support for clinicians and parent educators to include relationship enhancement programs for the benefit of parents and their children.