International research has evidenced the psychological impact of the COVID-19 pandemic on families, and the key role played by parenting stress levels. Although significant associations with parents’ past trauma and resilience have been shown, this study aimed to explore their complex interplay on the relationship between parents’ peritraumatic distress due to COVID-19, parenting stress, and children’s psychopathological difficulties. We recruited 353 parents with children aged two to 16 years via an online survey during the Italian second wave of COVID-19. Parents’ peritraumatic distress due to COVID-19, parenting stress, past trauma and resilience, and children’s psychological difficulties were assessed through self-report and report-form questionnaires. Parents’ past traumas significantly predicted peritraumatic distress due to COVID-19 and children’s psychological difficulties. The relationship between past traumas and children’s psychological difficulties was serial mediated by parents’ peritraumatic distress and parenting stress. Direct and total effects of parent’s resilience on parent’s peritraumatic distress were not significant, but there were significant indirect effects via parenting stress and via parents’ peritraumatic distress and parenting stress, indicating inconsistent mediation. This study evidenced the key risk and protective role played by, respectively, parents’ past traumas exposure and resilience on the relationship between parents’ psychological difficulties due to COVID-19, parenting stress, and children’s psychological difficulties, with important clinical implications.