There is loads of good information at www.childline.org.uk but specifically The Calm Zone has lots of games and activities, including yoga and breathing exercises. The games are designed to help ease worries and are particularly good for anxiety.
The Young Minds website https://youngminds.org.uk/ has so much information for children and parents and has a page dedicated to mental health concerns specifically related to coronavirus.
Mental Health Foundation
This long-established UK charity has set up a special page, https://mentalhealth.org.uk/coronavirus, specifically to offer advice to people during the coronavirus outbreak. It covers aspects such as parenting during the outbreak and talking to your children about the pandemic.
Mind has a dedicated page offering advice and answering questions that young people might have about the coronavirus, https://www.mind.org.uk/information-support/for-children-and-young-people/coronavirus/coronavirus-and-your-wellbeing/
The NSPCC has loads of information to help parents and carers and are offering advice on how to support children who may be anxious at the moment, as well as children with existing mental health conditions, https://www.nspcc.org.uk/keeping-children-safe/coronavirus-advice-suppport-children-families-parents/
Keep Your Head
This website, https://www.keep-your-head.com/cyp, signposts all the services available locally and is for adults as well as children, although there is a specific page for children’s services and resources. This site may be particularly useful for people who feel they need additional support.
I hope this helps. I'm happy to look for more but didn't want to overwhelm people so aimed for quality over quantity.
This is the coronavirus specific page...