I mean on a UNIX OS you could make it yet another group the user needs to be part of. Like the group for access to optical media or for changing network credentials. Whether the child gets root access is on the parent, but that is like with anything else. A child can get around this, but it means finding and exploiting a 0-day on the OS. If they are able to pull this of I would congratulate them.
There is a huge attack surface for this. For example, kid manages to buy an old phone. Resets the phone and creates an account. Kid buys something like a Pi 3 manages to get a regular phone to become an access point. Etc. If a laptop is not completely locked down, a kid might boot a live USB stick.
Barriers like that for accessing 18+ sites would be so much better than nothing.
And cheat devices can be taken away as soon as the parent notices them.