Kids shouldn't even be on social media, but at least the corporate ones are covering their ass against lawsuits well enough that they try to moderate content.
The Fediverse is not a place for kids. Servers catering especially towards kids are DEFINITELY not for kids, because that's exactly the kind of server I would build if I were a pedo.
The legal requirements for hosting content for kids are a massive headache that you definitely don't want to take on as a volunteer. The Fediverse can't even comply with the GDPR, let alone COPPA and its many international alternatives that actually see enforcement.
Of course I was a kid on the internet too and very few websites care about lying about your age, but if you do that and see the occasional dick, fetish porn or gore, you've only got yourself to blame. Plus, the Fediverse is full of misinformation, lies, and propaganda, from every side of the spectrum. Moderators can only do so much, and some moderators straight-up post misinformation and propaganda themselves. Best not to expose kids to any it that shit until their brains have developed a bit more.