Background: We assessed the mental health of children and adolescents aged 7 to 17 and their parents immediately after the first wave of the COVID-19 pandemic. Method: A survey online was conducted in Belgium between May 29th and August 31st 2020. Results: (1) Anxious and depressive symptoms were self-reported by one out of four children and hetero-reported by parents for one out of five children; (2) Anxiety scores were higher in children compared to adolescents; (3) Social introversion scores were significantly higher in adolescents compared to children; (4) No significant differences for anxio-depressive symptoms were found between healthcare worker parents and the other parents. Children's self- or hetero-reported symptoms were not found to be connected to parents’ professional activities. Conclusions: This cross-sectional survey adds evidence to the impact the COVID-19 pandemic has had on children's and adolescents’ emotional state, in particular on their levels of anxiety and depression. ; SCOPUS: ar.j ; info:eu-repo/semantics/published