The demands arising from the combination of work and family roles can generate conflicts (work-family conflicts), which have become recognized as major social determinants of mothers' and fathers' mental health. This raises the question of the potential effects on children. The current study of 2496 Australian families (7652 observations from children aged 4-5 up to 12-13 years) asks whether changes in children's mental health corresponds with changes in mothers' and fathers' work-family conflicts. Using longitudinal random-effect structural equation models, adjusting for prior child mental health, changes in work-family conflict were examined across four adjacent pairs of biennial data waves. Children's mental health deteriorated when their mother or father experienced an increase in work-family conflict, but improved when parents' work-family conflict reduced. Results held for mothers, fathers and couples, and the key pathways appear to be changes in children's relational environments. These results contribute new evidence that conflicts between the work-family interface are powerful social determinants of mental health which have an intergenerational reach.