I wish I knew where to find this now, but I once read a long story/account of a woman whose child was killed accidently on her elderly saint of a horse. The girl basically died right in her moms arms, and the mother was speaking out about helmet usage in children.

It's one thing for an informed adult to know the risks of not wearing a helmet and still doing without, but you don't put a 4 yr old with no balance on a 1200lb animal without a helmet. I don't care how good your horse is.