I gave my grandson his first bow when he turned 2. He loves it. Little fiberglass bow that only pulls about 10 pounds.
Some days he only shoots 2 or 3 arrows, then he's done. Some days he will shoot for 20 minutes or so. I don't worry about his form, or much of anything else except to try and help him learn how to hold and pull it. I just want him to have fun shooting.
I just finished an osage selfbow, and was going to give it to him for his 3rd birthday, but he's not quite big enough for it yet. Maybe by Christmas........
If they can pull it, let them start. They can learn the finer points as they get older...just get them out there and make sure they have fun.