Jump Start Jonny has free workouts and activities to get children moving.
https://www.jumpstartjonny.co.uk/free-stuff
Go Noodle has lots of content to support children in getting active indoors. They also have short videos on mindfulness.
Cosmic Kids Yoga is a fun way for children to experience yoga. There are themed sessions such as Star Wars, Pokémon, Frozen, The Gruffalo and lots more.