If it did happen, it would be better as an isolated server which occasionally has other servers added (using an opt-in federation system rather than the default one where everything federates until it's blocked).
But personally, I don't think children should be using social media without parents being next to them the whole time. (But that's just my personal view.)
By the way, the Fediverse in its current form isn't really meant as as a messenger. It's not secure enough yet.