The COVID-19 pandemic negatively impacted the mental health of children, youth, and their families which must be addressed and prevented in future public health crises. Our objective was to measure how self-reported mental health symptoms of children/youth and their parents evolved during COVID-19 and to identify associated factors for children/youth and their parents including sources accessed for information on mental health. We conducted a nationally representative, multi-informant cross-sectional survey administered online to collect data from April to May 2022 across 10 Canadian provinces among dyads of children (11–14 years) or youth (15–18 years) and a parent (> 18 years). Self-report questions on mental health were based on The Partnership for Maternal, Newborn & Child Health and the World Health Organization of the United Nations H6+ Technical Working Group on Adolescent Health and Well-Being consensus framework and the Coronavirus Health and Impact Survey. McNemar’s test and the test of homogeneity of stratum effects were used to assess differences between children-parent and youth-parent dyads, and interaction by stratification factors, respectively. Among 933 dyads (N = 1866), 349 (37.4%) parents were aged 35–44 years and 485 (52.0%) parents were women; 227 (47.0%) children and 204 (45.3%) youth were girls; 174 (18.6%) dyads had resided in Canada < 10 years. Anxiety and irritability were reported most frequently among child (44, 9.1%; 37, 7.7%) and parent (82, 17.0%; 67, 13.9%) dyads, as well as among youth (44, 9.8%; 35, 7.8%) and parent (68, 15.1%; 49, 10.9%) dyads; children and youth were significantly less likely to report worsened anxiety (p< 0.001,p= 0.006, respectively) or inattention (p< 0.001,p= 0.028, respectively) compared to parents. Dyads who reported financial or housing instability or identified as living with a disability more frequently reported worsened mental health. Children (96, 57.1%), youth (113, 62.5%), and their parents (253, 62.5%; 239, 62.6%, respectively) most frequently accessed the internet for mental health information. This cross-national survey contextualizes pandemic-related changes to self-reported mental health symptoms of children, youth, and families.