Any kind of two-wheeled vehicle is inherently hazardous and no one will tell me any different.
My late father-in-law was as sensible a man as you could wish to meet, and anything but reckless. When he went out on his motorbike he always had his full kit on and was covered head to foot in leathers, with a quality helmet. He rode with all due care, for example taking up a car space in traffic and never filtering.
Nonetheless, he eventually had an accident. It was not his fault, but that was no consolation when he was lying in a hospital bed with multiple injuries. His wife was told in plain words that he was unlikely to survive.
Fortunately, he did, but he was in far from A1 condition for the rest of his life. Never rode again; he actually bought a Reliant Robin! (He never got a car licence so it had to be a three-wheeler.) That kept him mobile for a bit, but eventually, he had to give up driving.
Someone who rides any type of two-wheel vehicle at high speed (and high speed in this context is not that fast) without any protection for their head is dicing with death. Your head doesn't need to be going very fast when you stick a nut on a tree or a lamppost or the asphalt of the road. Chances are you are a goner. Scraping your limbs and arse along the tarmac can have some pretty unpleasant effects too; it really isn't recommended.
To combine a lack of protection with recklessness is the height of folly. I don't honestly blame the kids. They are kids and when you're a kid you often do stupid, reckless things. I blame the parents for being so indulgent, and for being so dense they can't foresee the potential dangers. You might as well give the kids a handgun and encourage them to play Russian Roulette.