Objective To identify interplay of early maternal and paternal mental health symptoms for predicting child mental health and development. Background Research on family mental health has largely excluded fathers, although the well‐being of both parents is likely to be important for child development. In this study, we analyzed (a) intrafamilial dynamics between mothers' and fathers' early mental health symptoms and (b) the importance of separate (mother and father) and joint (additive, hierarchical, and buffering) theoretical models of parental mental health for predicting child mental health and development. Method Finnish mothers and fathers (N = 763), half of whom conceived through assisted reproductive treatments (ART), reported their symptoms of psychological distress and depression from the pregnancy to 2 months and 12 months postpartum. Later, when the child was 7–8 years of age, parents (N = 485) reported the child's internalizing and externalizing symptoms and social and cognitive developmental problems. Results We identified both co‐occurrence and compensation in intrafamilial early parental mental health. Further, mothers' symptoms alone (separate mother model) predicted child internalizing symptoms, whereas joint parental symptoms (additive model) predicted problems in executive function. Conclusion The pre‐ and postnatal mental health of mothers and fathers is important for later child development. Implications To support healthy child development, both parents need to be screened for early mental health problems, and psychological help should be offered to families across the pre‐ and postpartum period.