Parental controls can set browsers in "child mode" where the browser sends an "I am a child" header to the server and social networks etc. need to honour it. This has existed for twelve years already: https://blog.mozilla.org/netpolicy/2014/07/22/prefersafe-mak... . It can probably be amended with a more granular set of levels, but that would be the best way forward.

The problem of "parents are negligent" is also solved by existing laws which have fines for parents who are negligent towards their children, and governments absolutely love collecting fines, so all the incentives are properly aligned.