If you want to use it for fun, someone suggested a Luanti server, and you can also find a lot of really easy to set up game servers using a tool like LinuxGSM. Most games they support require very minimal configuration if any.

If you want to use it for something educational, depending on the age range of the kids, you might be able to use it to teach computing skills. For example, you could set it up as a pubnix/tilde where kids can get shell access, basically a remote connection to the command line, and you or someone can set up some services for them. Pubnixes tend to provide things like chat over IRC, something like a forum maybe over something a lot like Usenet, a webserver where users can host their own webpage they had to design themselves, less commonly but good for the kids would be voice chat with something like Mumble, they often have an array of programming tools, collaborative tools like calendars or something akin to Google Docs… It could be a good place for technically inclined kids to learn about computers, servers, Linux, programming, basic web development, etc. All the services could be kept internal to keep strangers away from the kids, maybe remote access strictly through accounts only given in person to members. If you’re interested in something like that, I might be able to help. Feel free to DM me or check out the Tildeverse to get an idea of what sort of things tildes and pubnixes offer their users.