Just buy a Realm subscription from Mojang.

I ran a private server for my kids for a while and it was a huge headache. Bedrock is purposely made to not make it easy to DIY. It was difficult for their friends to join, everyone was mad when it was down when they wanted to play, and I had to take great care to not lose their progress.

Paying for a Realm doesn't scratch that DIY itch, but your kids will actually get to enjoy playing, which is really the point, isn't it?

Alternately, run a Java server and use the Java-to-Bedrock plugin / proxy / adapter.