<p>The COVID-19 pandemic and related confinement measures had profound impacts on the mental health of children and adolescents. This study investigated familial mechanisms underlying such impacts by studying the case of Singapore, which has undergone a period of local transmission without many restrictions followed by a nationwide lockdown in 2020. In June 2020, we collected retrospective reports on everyday activities, social relationships, and psychological distress of 164 children and adolescents (<i>M</i>_age=14.3y, range = 7–18 years) across three timepoints: pre-pandemic (January 2020), pre-lockdown (February to March 2020), and lockdown (April to May 2020). Parental psychological distress, parenting values and practices were also measured. Multilevel modelling showed increased psychological distress in children from pre-pandemic to pre-lockdown, and from pre-lockdown to lockdown. Mediation analyses indicated that the increase in children’s psychological distress from pre-pandemic to pre-lockdown was partially explained by a decrease in time spent outdoors, an increase in their conflict with fathers, and an increase in their mothers’ psychological distress, with the last being the only significant mediator when all three were considered simultaneously. The increase in children’s psychological distress from pre-lockdown to lockdown was partially explained by a further increase in mother’s psychological distress. These findings suggest that multiple individual and familial factors may underlie COVID-related deteriorations in children’s mental health.</p>