This longitudinal study assessed how parent-child relationship quality during the first COVID-19 lockdown was related to changes in internalizing, externalizing, and sleep problems during the first months of the pandemic: during lockdown, partial deconfinement, and total deconfinement. Participants included 167 children (9–12 year) and their parents recruited in the province of Quebec, Canada. Child behavior problems decreased between lockdown and the two deconfinement assessments, but more sleep and behavior problems were associated with lower levels of relationship quality (more conflict, less closeness, and more insecure attachment). Significant interaction effects showed that changes in externalizing and sleep problems varied as a function of parent-child relationship. Results support the critical importance of the parent-child relationship with regard to child adjustment in middle childhood in times of crisis such as a pandemic. They also highlight resilience in children aged 9 to 12, with a decrease in problems over time.