True hacker kids will figure it out no matter what. If the phone is restricted they will jailbreak it or beg an old one from a friend. If the network is locked down they will crack the password for the neighbor’s wifi. If the clearnet is locked down due to authoritarian laws, they will end up on incredibly sketchy sites on the dark web. If you block all escape outlets they will do really stupid things in nihilistic protest like huff cans of whipped cream or scam money from crypto. The power of bored youth springs eternal.
What I’m saying is you can set rules, you can try your best, but under no circumstances can you build an impenetrable wall for determined kids. Things like this header solution or better controls on the end device would make things safe for the vast majority of kids. So don’t ruin the internet for adults because of a handful of unruly kids who are going to get in trouble no matter what.
Walls don't deny access, they change traffic patterns. A header can be honored on a phone designed for a child and ignored by my phone. Can some kids get through, sure. But not the vast majority. If people were honestly doing this for kids, this would be the solution being pushed. This has nothing to do with kids, hence other types of solutions being pushed. Don't trust what people say, observe what they do.