I dunno, you have experience being a kid, right? Young kids are just not interested enough to look for porn, not to say figure out how to use VPN to access it. Lax restrictions like we have today are enough to stop porn from being forced on children who are not interested in it
It's not just "look for porn", it's "being exposed to stuff they shouldn't be exposed to".
E.g. the problem of social media is not that kid access information. It's more that kids get harassed by other kids.