alt Hacker NewsBack then, you had 20, 30, 50 'friends' on facebook and basically all the content you saw was made by them. Except for chatting, you could basically view all the 'daily content' (all the posts by everyone you had on your friends list) in maybe 10 minutes.
Then facebook turned to "let's show you random political articles instead of your friends dinner plates", and people moved to instagram... which stopped showing your friends dinner plates soon after it got bought out by facebook and it too replaced the friends dinner plates with random "reels".
If the kids only saw stuff posted by their 'friends', instead of being pushed a lot of random garbage they never decided to 'follow', it would still be a much nicer place.
I think you're conflating issues. There's the algorithm pushing garbage and then the general issues of social media itself - cyber-bullying, image crafting + social comparison, FOMO, and so on. The latter is the main driver of the negative effects in children. It's basically just taking all the normal negative issues that happen in a school environment and then bumping everything up exponentially.
Even consider your innocuous example of dinner pics. Kids are extremely insecure and prone to envy. Obviously some are going to be eating much more nicely than others on average. Think about the knock-on effects of that when suddenly that's being shoved in their face. And again that is for a behavior that you yourself offered as ostensibly harmless. In practice far worse things happen, and constantly.