A better solution is more robust device management, with control given to the device owner (read: the parent). The missing legislative piece is mandating that social media companies need to respond differently when the user agent tells them what to send.
I should be able to take my daughter's phone (which I own), set an option somewhere that indicates "this user is a minor," and with every HTTP request it makes it sets e.g. an OMIT_ADULT_CONTENT header. Site owners simply respond differently when they see this.