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.