This isn’t that hard to solve.

Kids have their own account on the computer.

Make it the law that if a computer account is set up as a kids account it must send some kind of not-adult header with all requests. Enforce that in the OS so it can’t be messed with.

Leave non-kid accounts alone.

Make it the default thing that happens if you set it up as a kids account so anyone can do it.

Thinking aloud here, but perhaps the answer is in the question: general-purpose computers get a free pass. Point being that kids generally don't use such things. Because otherwise I have exactly the same question as you. It's an existential question for software freedom.