The COVID-19 pandemic has led to increased mental health concerns, including depression and anxiety among parents and internalizing and externalizing problems among youth. To better understand the mechanisms and moderators of child mental health during the pandemic, the current study tested two moderated mediation models in which parent depression and anxiety indirectly impacted child internalizing and externalizing problems through negative effects on multiple parenting variables, with these associations moderated by families’ exposure to COVID-19-stressors. A national sample representative of U.S. parents (N = 796, 48.2% female, Mage = 38.87 years, 60.3% Non-Hispanic white, 18.1% Hispanic/Latinx, 13.2% Non-Hispanic Black/African-American, 5.7% Asian, 2.8% Other Race) completed a cross-sectional online survey in February-April 2021. Children ranged from 5–16 years old (Mage = 10.35 years, 59.8% Non-Hispanic white, 17.2% Hispanic/Latinx, 13.7% Non-Hispanic Black/African-American, 4.5% Asian, 4.8% Other Race). Parent depression/anxiety was directly and indirectly associated with child internalizing and externalizing problems. For both internalizing and externalizing problems, indirect associations occurred by means of increased parent hostility and inconsistent discipline and decreased routines and parent supportiveness. There were also specific indirect effects through decreased monitoring (internalizing problems) and parenting self-efficacy (externalizing problems). Multiple indirect effects were moderated by number of COVID-19-stressors experienced. Notably, COVID-19-stressors did not have direct effects on child mental health when other variables were considered. Findings highlight the buffering effects of parents for child mental health, the need to address parent depression/anxiety in child interventions, the utility of existing evidence-based parent interventions during the pandemic, and the need to assess families’ level of exposure to COVID-19-stressors.