A family we know lost an 8 year old in a UTV accident about 15-16 months ago. It was such a horrible tragedy. A helmet might well have saved her. I wouldn’t consider letting a child on an ATV, UTV, go cart, mini bike, etc... without one. Yeah they’re hot and a pain in the butt, but it’s just better safe than sorry.
I started riding motorcycles about 12, and my mother swore she’d take it away from me if she ever caught me without a helmet (she would’ve too). To this day I wouldn’t ride a motorcycle without one. We made our kids wear them on ATV’s til they were 18 - they still do if they’re playing, not if they’re checking cows. Their choice. But no I don’t wear one on atv’s or utv’s on the farm either - but if we had little kids around and that’s what it took to convince them to, I’d do it.