The COVID-19 pandemic has been linked to a range of behavioral problems in children. To date, however, longitudinal studies with data prior to the pandemic are rare, and moreover, few studies have examined the family context. This is notable as evidence suggests that mothers were highly vulnerable to the effects of the pandemic, and stress proliferation models would argue that children’s wellbeing are undoubtedly affected by maternal wellbeing. In the current investigation, we examine changes in maternal depressive symptoms and children’s behavioral problems from prior to the pandemic to the first few months of COVID-19 in the U.S. The results suggest a significant increase in children’s internalizing problems and maternal depressive symptoms. Consistent with stress proliferation models, the relationship between COVID-19-related stressors and children’s behavioral problems were mediated by maternal mental health.