Are they posting as well or just consuming? I wouldn't say that there's anything wrong with them sharing it, because the pitfalls you would get from that could happen from them having individual accounts anyway.
The thing that stood out to me immediately though would just be their digital footprint and how once they post something publicly, literally anyone in the world can see it. Even if their account is private, the people they let follow them could still share the content outside of the app. It's also really important to make sure that they don't reveal their exact location, school etc by accident. You'd be surprised how little it takes for someone to deduce where another lives.