My one brother let his son drive an ATV -- in the backyard. Only. In many places their not street-legal. Even then the kid had to wear a helmet and he couldn't, obviously go very fast. Oh, yeah -- the kid was 15.
The other brother let his son drive the ATV. In a circle. In first gear. Helmet, of course, and other gear. The kid, 11, loved it, even though it was at about 1.5 miles per hour.
Neither would let either kid do it alone. Ever. And especially not at night.
I'm sorry, Alison, and I understand your grief. But this is a case of stupid parenting.
Kat -- I agree with you about the guns and hunting, although I'd allow it IF the kids were accompanied by an adult AND had completed a safety course.