<p>Studies examining pandemic-related changes in children and their families’ psychosocial functioning have produced mixed results and little is known about how these changes interrelate. The objectives were to (1) examine changes in parent and child mental health and behaviors in Canada following the first wave of the COVID-19 pandemic, and (2) identify potential protective and risk factors of these changes. Objectives were examined in 426 families, with children aged 7–9 years, from the 3D-Transition longitudinal Cohort. On average, child mental health symptoms and social difficulties increased during COVID-19. However, this depended on the child’s starting levels of symptoms: children that were doing well before the pandemic did significantly more poorly, while children that were having more emotional and social challenges did significantly better. On average, parents reported less access to social support networks and more stress during the pandemic, but no significant changes in their parenting practice. Increased parental stress and decreased child nurturing during the pandemic were independently and significantly associated with increases in children’s internalizing, externalizing, and social difficulties. Decreased access to parents’ social support networks and increased restrictive parenting also explained increases in some child symptoms during the pandemic.</p>