Mind, the mental health charity, have specific advice relating to mental health and the pandemic, including this article about speaking to teens.
The Children's Society
https://www.childrenssociety.org.uk/The children’s society works to improving the lives of children across Britain, here’s some of their advice on young people and anxiety.
Young minds are a charity dedicated to the mental health of young people – they have lockdown specific advice, like the information in this article here.
The NSPCC have some information about children's mental health- here's a link to some of their information on depression, anxiety and mental health.
The NHS also has a dedicated advice page with some helplines for further support.