BACKGROUND: Parents and their children with chronic physical illness (CPI) are often predisposed to experiencing psychopathology, a phenomenon, which may be exacerbated during the COVID-19 pandemic. OBJECTIVE: This study modelled trajectories of psychological distress experienced by children with CPI and their parents before and during COVID-19. METHODS: Overall, 134 parent-child dyads enrolled in an ongoing cohort study, Multimorbidity in Children and Youth across the Life-course study, participated in this COVID-19 substudy. Data were collected at three time-points: pre-pandemic (Dec 19-Mar 20) and two intrapandemic periods (Dec 20-Mar 21 and Jun 22-Oct 22) in Ontario, Canada. Parents reported on symptoms psychological distress they and their children experienced, using the Kessler-6. Linear mixed-effects modelling was used to estimate trajectories of psychological distress over time. RESULTS: Parents of children with CPI experienced increased psychological distress from pre-pandemic to intrapandemic 2021, which declined slightly by intrapandemic 2022 (B CONCLUSIONS: Children with CPI and their parents experienced elevated psychological distress during COVID-19. Understanding the course of psychological distress for vulnerable children and their families is crucial to informing healthcare strategies that foster mental well-being and preparedness for future global crises.