This study explores the dyadic longitudinal interplay among parents' psychological distress, family cohesion, children's internalizing/externalizing behaviors, and peer-related social competence within individual and dyadic relationships. Data came from a nationally representative longitudinal cohort study of children in South Korea. The analyses included 1,779 families across three annual timepoints. Children were 4 years old (48.6% girls) and the mean ages of mothers and fathers were 34.8 and 37.3 years, respectively, at baseline. At each assessment point, mothers completed questionnaires regarding their psychological distress, cohesion, and their child's internalizing/externalizing behaviors, while fathers provided information on their psychological distress and family cohesion. Preschool teachers also evaluated children's peer social competence at each measurement timepoint. The results revealed bidirectional associations between maternal ratings of psychological distress and children's internalizing/externalizing behaviors as well as between maternal ratings of cohesion and children's internalizing/externalizing behaviors. Paternal ratings of psychological distress were longitudinally related to maternal ratings of children's internalizing behaviors. Teacher ratings of children's peer social competence were associated with maternal ratings of internalizing/ externalizing behaviors and paternal ratings of psychological distress and cohesion. Teacher ratings of peer social competence and maternal ratings of internalizing and externalizing behaviors were reciprocally associated. Maternal and paternal ratings of psychological distress and cohesion were prospectively and bidirectionally associated. The findings demonstrate prospective transactions among family subsystems, some of which were moderated by child sex, while highlighting the importance of obtaining data on each family member and considering interactive effects of both parent and child sex in this line of inquiry.