If you're using something like a fire tablet and you set them up as a kids account that's not how it works. If you next through everything your kid cannot play minecraft online, not even on your own little private server.

Getting an actual kids account to work online with minecraft involves setting the right permissions across 2-4 websites and 1-3 companies. I think it took me around 4 hours of trial and error to get it working.

I might be wrong about this, but at least in my experience you just can't "next next next." There's too much complexity!

I'm essentially the maintainer of a series of accounts for each kid, these days. Woe unto anyone without a password manager!