>> SteamOS could still be affected

Steam itself does age verification, which when you first boot a steamdesk, afaik it forces you to log into steam before you can do much of anything without some initial hackery. That said, once in there's nothing stopping them from launching into desktop mode, launching firefox, and watching pr0n that way.

Sadly the solution is still for parents to do real parenting, but that's like saying stupid people shouldn't breed.