That's still treating the symptom. Regulating the entire Internet's platforms would end up creating a surveillance state. And the big companies would find some way to worm out of it, or simply treat the financial consequences of breaking the laws as a cost of doing business.
Parental supervision and content filtering can and should be solved on the client-side at the OS/app-level, entirely locally. No user info like age ever needs to be sent anywhere, and it doesn't even need to be collected. The reason smartphones can't do this right now is because these companies are deliberately making the client-side filtering solutions difficult to implement and use, as explained in a past comment: https://news.ycombinator.com/item?id=49118578