A recent Pediatrics study of bicycle-related mortality rates in Ontario, Canada, from 1991 to 2002 among bicyclists 1 to 15 years of age set out to determine the effect of legislation requiring minors to wear bicycle helmets.
They found that as children under 15 years of age were required to wear helmets, the youth mortality rate from bicycle accidents decreased significantly.
I would throw in as an important adjunct that parents who strap the helmet on their kid and then don’t wear one themselves provide poor role model behavior to their kids. Adults are prone to all sorts of accidents that may have little to do with skill and more about happenstance (or even just ‘riding with kids, who are less predictable’). Concussions are no fun, and even non-serious head knocks can be awfully bloody. Watching the adult role model bleed profusely from a head wound is unlikely to be a great formative experience for a child. Or so I’d guess. I’d suggest avoiding field study of this, anyway, per my mother’s classic rule: Blood belongs inside the body.