> In some countries you can even do that on public roads!

I might be wrong but in the US I think it's generally anything goes on private land. Public roads would be the only relevant thing to consider.

What prevents absurd situations is (IIUC) the combination of child labor laws and the need to keep your insurance policy affordable.

I suppose if a parent turned his toddler loose in an excavator he might get brought up on some sort of child abuse law but honestly I doubt it. Some of the people out in the sticks teach their kindergartners to wield a shotgun and the government seems to leave them alone.