Way back when, probably when I was abround 5 I started shooting a bow. It was a fiberglass recurve with a draw somewhere around 15 lb. Way before compounds came out. My uncles archery hunted and there was always hay bales set up behind my grandparents house. They taught me the basics and I shot almost every day. And I carried that bow and a quiver full of arrows almost all the time running around the woodlot shooting at opportune targets. Stump shooting they call it now.
I say get them out and start them off as soon as you can. And until they get the proper form down, be sure they wear those arms guards. That string slap can really hurt those little forearms. If you're unsure about how to instruct them, I'm sure your local archery shop can show them. Its good for business.