I also always wore a helmet when jumping, my dad made me (this was back before 'real' helmets were available - he ordered a jockey's helmet from the Tbred Racing Assn in NYC.) This was mostly because a friend of mine was permanently and severely brain damaged when she tried to jump her hot Anglo-Arab over a picnic table wearing shorts, tennis shoes and no helmet . . . !
We're all fine, but there were a certain number that didn't make it, and if you knew them, that was bad.
With that said, I think the precautions taken today are beyond silly. My kids swim with a buddy, wear helmets on horses and bicycles, and practice extreme gun safety . . . but other than that, I simply ask them to use common sense.
I did a lot of stupid things on horses when I was a kid. I certainly don’t want my kids doing what I did! Fun, yes, but I was a complete idiot. I would kill the kids myself if I ever caught them jumping over sharp objects, for just the reason you mention. Trash themselves if they must, but the nearest horse ER is an hour away (Marion duPont Scott) and there are extreme limits on what can be done to fix a busted horse, as witness Barbaro.
It’s sobering when you see a guy who used to be huntsman at a prominent hunt and a steeplechase rider driving his motorized wheelchair up and down the road. (You may know who I mean since you hunt and the hunting world is small.) He didn’t hit his head, but broke his neck. Unfortunately even a $400 GPA won’t prevent that, and I wish I could think of a way to prevent such neck injuries (a helmet with springs attached to your shoulders??).