Do you think there could be explicitly child-friendly social media? I don't know how you'd get past the network effect, but could it work? Maybe adults would have to set up groups, and connections could only be established with Bluetooth or something to ensure physical proximity (yes you could cheat but you'd need weird hardware or software to do so).
Adults setting up groups would be so that you could have a group for "year 11 maths class" but not for "people who hate susan"
It'd be fine if they were just using chat apps. It's not the communication that's the problem, or the bullying. That's going to happen regardless, and it's nothing new to online spaces. If anything, having online spaces available is helpful, because kids who are excluded at school are able to find places they fit in online. It's the algorithmic feeds and short-form video content. It's flooding their brains in their most formative time.