I doubt it matters because nobody seems to configure the parental controls that are there, anyway. Only schools seem to try, and only because they're legally held responsible for what kids do on their computers.
You can set up, for example a child friendly DNS (Family version of OpenDNS) then Firefox come along and bypass it (DoH).
Probably, we need an OS level setting, per account, browsers would need to expose it to websites, apps installed would have to check and provide content according to local legislation/rules. Bypasses would have to be performed by an "adult" account.
The main issue is that adults would be able to set themselves as children and avoid negative commercial activity. That's a problem because it means FANG, etc, will lobby against it and then ensure their implementations are convoluted and broken.
Parents should be teaching kids from a early age how to use technology responsibly.