Purpose <p>The purposes of this study were to: i) identify factors associated with children’s 24-h movement behaviours and mental health indicators at the end of the first wave of the COVID-19 pandemic in Ontario, Canada; ii) explore parents’ strategies to cope with the challenges of the pandemic and promote adherence to the 24-h movement guidelines among their children.</p> Methods <p>Parents of children aged 5–13 reported their eldest child’s movement behaviours and demographic characteristics, and mental health. Open-ended questions were available for parents to report strategies to promote physical activity (PA), sedentary behaviour (SB), and sleep before and during the pandemic.</p> Results <p>The results suggest that children (<i>n</i> = 271, M_age 9.7± 2.5&#xa0;years, 55% boys) mostly engaged in adequate levels of PA (86.3%), but also in excessive SB (80.4%) and insufficient sleep (52.8%). Age (OR = 0.86, <i>p</i> &lt; 0.05) and presence of an electronic device in the bedroom (OR = 0.32, <i>p</i> &lt; 0.05) were inversely associated with odds of meeting the SB guidelines. No other significant associations were observed with PA, SB, sleep, or mental health indicators (<i>p</i> &gt; 0.05). Strategies used by parents to promote adherence to the 24-h movement guidelines changed from before the pandemic period to the end of the first wave, with a shift from organized activities such as sports and school-related activities to outdoors and family-based activities.</p> Conclusion <p>Families adapted how they engaged in PA and SB during the pandemic by increasing their family activities and unstructured and outdoor play. Yet, behaviours were not significantly associated with the mental health of their children.</p>