If we did build it and it became popular, it would quickly be taken over by the same forces that are destroying the current internet. To get good social media sites (and a better internet as well), you would first have to change the economics of the entire system driving these forces. But as is said "It's easier to imagine the end of the world than the end of capitalism".
it doesn't need to be social media, it could be an entirely closed system with some moderators curating content.
It's really not that dramatic. Just build it like more classic media. Curated content the company takes responsibility for, closed platform, pay upfront. Or have public programming, that is the oldest model there is.
Ad driven online content is especially bad for kids. But let's not pretend the only way to find an alternative is to end the world.
The fact is the "bad" solution is popular because consumers say they care about these things but then in real life they act like they don't. If no one watched the problem would solve itself. Thus, I'm not sure the solution is even to be found in platforms, if parents are burned out or don't have ways to make better choices for their kids.
That's a reason for these laws, to essentially just take it out of people's hands.