My two boys shoot all the time. They started at age two. I know on other posts I mentioned the "under 3 longbow" class that the local bow club started for them in tournments. They now both have a "real" bow. My oldest (5yo) shoots a Protogy and my youngest (3yo) is getting a Maddog bow built right now.
You gotta get them kids doing something besides getting into trouble!! The things they start now will stick with them. I am not saying they will always shoot a bow, but they will learn hard work, persistance, endurance, and the satisifaction of them all. My oldest can shoot a soda can most every shot at 10-12 yards. When he first shot one, you would have thought it was a 200" B&C monster!! They get a real since of accomplishment.
Great for their self esteem, not to mention the quality time with Dad.