So it’s a bad idea when parents do it, but a good idea for the government to?

I can't believe I need to spell this out but here goes.

If kids are generally interacting with each other on social media, then excluding your own child from that will make them a pariah. You know, like the kid that can't go on the school camp because reasons.

With the recent ban, kids are no longer "generally interacting" with each other on social media. It doesn't matter that some will inevitably circumvent the ban. This gives parents the opportunity to enforce boundaries.

it’s their role here, not the federal governments.

Nonsense. Federal government's impose age restrictions on all sorts of things for a variety of reasons. There are legislated ages of consent, alcohol consumption, driving, et cetera.

So what? Most of the shittier stuff we pass does.

If something has bipartisan support then more or less by definition, you can't argue that "we" didn't ask for it, because everyone's representatives are asking for it.