This study aims to describe parents' perception of short-term effects of the COVID-19 pandemic on their children's emotional well-being, difficulties, and aggressive behaviour in German-speaking Switzerland. Results from the first wave (baseline) of an opt-in online survey on the emotional well-being, difficulties, and aggressive behaviour of children aged 4 -14 years during the first nationwide lockdown in spring 2020 are presented. Standardised scales were used to survey children's aggressive

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... behaviour (FAVK) and emotional and behavioural difficulties (SDQ). Up to June 8, 2020, 159 parents of 231 children (aged 2 -14) participated in the study. Results indicate that according to their parents, most children were able to cope well with the lockdown. However, respondents reported difficulties for two fifths of the children. Although the difficulties were not severe in most cases, the children's emotional and behavioural problems were a burden for most families. Further, compared to the norm sample of children (aged 4 -14), children with clinically relevant aggression scores were overrepresented in the sample. Younger children (aged 4 -8) and boys seem to have been more burdened with factors that trigger aggression than older children (aged 9 -14) and girls. In line with other studies, our results indicate that COVID-19 containment measures had a negative impact on the children's well-being and behaviour from an early stage. These difficulties represent a developmental risk for the children as well as for their families as a whole. However, the pandemic affected different families in different ways. For example, children of different ages were affected differently, and there was an association between the children's difficulties and the parents' organisation of childcare and remote work from home. As long-term effects of the pandemic on children's development are to be ex-