I found this thread by accident whilst looking for something else. However, I am relieved that I am not alone in this issue. I am in the UK where there are no helmet laws - even though mant parents and kids wish there were.
My girls are age 3 and 6. One has a bike with training wheels, the youngest has a trike. They both have helmets firmly fastened to their head EVERY time they ride. My eldest was about 4 and was on the trike that my youngest now has when it tipped and she went flying. Her head hit the kerb! That night we bought her a helmet.
She would not now dream of getting on her bike without it and my younger one sees this and wants to be the same anyway. If this was not the case, they would not ride until they accepted that they had to wear them!
In the UK, kids ride bikes without helmets, skate without them and even ride horses without them (although it is the law for under 14 now to wear them riding horses on the road) Many kids do wear them but just as many dont and our injury rates are not good. A child I know is in intensive care right now with severe brain injury from a bike accident and no helmet.
My kids know that I would never dream of riding my horse without a helmet and so there is never any debate about it on their ponies and the same applies to bikes and trikes. Even though other kids dont and the law is not in place, no helmet, no bike riding because as a mum I would not forgive myself!!