There isn't, the only posibility is creating an own instance of Mastodon or whatever, where you can implement the corresponding rules and filters for kids. The fediverse as such can't never be safe for kids, due to it's own structure if safe instances are federated with such which are not family safe. https://blog.elenarossini.com/gotosocial-empowers-you-to-have-your-own-home-on-the-fediverse-with-unique-controls/

Alternatively social media, specific for kids

https://www.internetmatters.org/resources/social-media-networks-made-for-kids/

It is always needed to educate the kids about the risks and for an correct and safe behavior in the network, using safe search engines, eg.SwissCows, private and family safe by design.

What do you expect that to do, on a technical level?

Because if all you want to do is the societal result, you can use any existing backend and configure it to disable federation except with other servers with the same goal. Unclear what you are trying to achieve.