The Red Cross have released First Aid Champions which helps parents teach their children about first aid.
The resources have been adapted to use at home and the suggested pathway of using the First Aid Champions work is here: https://firstaidchampions.redcross.org.uk/primary/guidance-and-support/home-learning-pathway/
Please read this guide first, as it will allow you to decide if it is suitable for your child. It is designed for 5-11 year olds, but you may only wish to look at some aspects.