A good start is Google (of course). We tried “how your body works” and got dozens of good looking sites. This American one is pretty comprehensive - http://kidshealth.org/index.html -
It has separate sites for parents, kids and teens. The kids’ site in particular has animated films to show how things like muscles, the circulation or the liver work. It’s pretty easy and informative - http://kidshealth.org/kid/htbw/htbw_main_page.html - will take you directly there
Books too of course – a very quick search found, for example, How Your Body Works by Judy Hindley and Colin King.