How do you set up a Windows 11 machine for your kids?

You can make them a limited user account and enforce parental controls (both in the OS and maybe on the router) if you really want to. Don't be surprised if they find workarounds...

Back in my day, my peers and I just had unfettered root (well, admin) access to our machines. I dunno if it's realistic to protect kids from all the things anyway. Whatever you shield them from is just going to seem even more appealing, and no doubt their friends will find it for them if they really want it.

Give the machine to the kid and let the kid figure it out for themselves

How old are the kids? What do they want to do?

Setting things up for a 6 year old who wants to watch YouTube videos (hint, install FreeTube) is different than setting things up for a 16 year old.

Not? Ubuntu, or Mint? Why do they need Windows, specifically?