BACKGROUND: The COVID-19 pandemic has significantly changed the lives of children and adolescents, forcing them into periods of prolonged social isolation and time away from school. Understanding the psychological consequences of the UK's lockdown for children and adolescents, the associated risk factors, and how trajectories may vary for children and adolescents in different circumstances is essential so that the most vulnerable children and adolescents can be identified, and appropriate support can be implemented. METHODS: Participants were a convenience sample of parents and carers (n = 2,988) in the UK with children and adolescents aged between 4 and 16 years who completed an online survey about their child's mental health. Growth curve analysis was used to examine the changes in conduct problems, hyperactivity/inattention, and emotional symptoms between the end of March/beginning of April and July using data from monthly assessments over four months. Additionally, growth mixture modelling identified mental health trajectories for conduct problems, hyperactivity/inattention, and emotional symptoms separately, and subsequent regression models were used to estimate predictors of mental health trajectory membership. RESULTS: Overall levels of hyperactivity and conduct problems increased over time, whereas emotional symptoms remained relatively stable, though declined somewhat between June and July. Change over time varied according to child age, the presence of siblings, and with Special Educational Needs (SEN)/Neurodevelopmental Disorders (ND). Subsequent growth mixture modelling identified three, four, and five trajectories for hyperactivity/inattention, conduct problems, and emotional symptoms, respectively. Though many children maintained 'stable low' symptoms, others experienced elevated symptoms by July. These children were more likely to have a parent/carer with higher levels of psychological distress, to have SEN/ND, or to be younger in age. CONCLUSIONS: The findings support previous literature and highlight that certain risk factors were associated with poorer mental health trajectories for children and adolescents during the pandemic.