There's no reason to hold the parents culpable. It would be up to the device manufacturer to ensure that this isn't possible on a system that has parental controls enabled. This is already a solved problem - see how MDM solutions do it, and see Apple's ban on alternative browsers.

It's not even necessary to block parents from giving their children Linux desktops or whatever. It'll largely solve the problem if parents are merely expected to enable parental controls on devices that have the capability.