You sound so certain that these other parents are being negligent. Despite the fact that my 9yo doesn't have a phone or tiktok, I'm going to argue the other side's case.

You're right "we don't know what it does to kids brains". But that means we should give parents discretion. Some parents want to run an experiment where they give their kids constant tiktok. Maybe we'll find out in a generation or two that this was bad, good, or a nothingburger.

I'm in my 50s. I have watched sooo many moral panics evaporate to nothing. TV, video games, D&D, heavy metal music, porn, literal satanism, and now everyone's favorite whipping boy, social media.

Let it go. Parent your own child, let other people parent theirs.