Objective: To investigate differences in movement behaviors (physical activity, sleep, screen time) in both parents and children during the early stages of COVID-19 pandemic in Australia, compared to pre-COVID-19 national data; and, estimate associations between these movement behaviors with parent and child mental health. Methods: We used cross-sectional baseline data from the COVID-19 Pandemic Adjustment Study (CPAS; N=2,365). Participants were parents of children aged ≤18 years, residing in Australia. We drew on nationally representative pre-COVID data from the Longitudinal Study of Australian Children (LSAC; N=9,438). In both studies, parents provided the same self-report measures of physical activity, sleep quality, as well as measures of child physical activity and screen time. Parents reported on their own and their child’s mental health. Results: Compared to LSAC, children in CPAS had more sleep problems (17.4% vs 8.9%, p<.001) and more weekend screen time (3.98 hours vs 3.35 hours, p<.001), while more parents had poor sleep quality (56.7% vs 21.0%, p<.001) despite increased weekly physical activity (3.86 days vs 2.85 days, p<.001). Children’s sleep problems were associated with increased depression, anxiety and irritability symptoms, after accounting for physical activity and screen time (all p<.001). Poorer parent sleep quality and lower levels of physical activity were associated with poorer mental health across all indicators (all p≤.001). Conclusion: Government funded mental health programs to implement evidence-based sleep interventions for children and their parents, along with targeted messaging around physical activity should be considered to promote mental health within the family context during lockdown restrictions.