In my house, no helmet, no bike, motorized or not. If I catch you without one, say goodbye to your bikes. Its been the rule since they've been old enough to ride. Since they've always had to wear them I don't get any grief. It drives me crazy to see families on outings and not wearing helmets. How can the kids learn if the parents don't wear them.
I think Mass has a kids under 12 helmet law but who knows if anyone enforces it.