In reply to ablackett:
Realms is good but might not be best suited for their ages.
With Realms, you pay for a private, always available online world and you can control who is allowed to connect.
Without Realms, you can still play "online" together, but one of the kids' devices will host the world, and that person needs to be online and the other kid joins them. So, one can play without the other online but not the other way around.
Given their age, I would start with the at home multiplayer. Pay attention to taking backups of their map in case they mess things up and get upset.
It's a great game. I used to really enjoy playing it - give it a go as well.
Something our kids used to love was Minecraft Build Battles. Join a shared world in creative mode. You need to set it up as a blank field with a HUGE wall in the middle. You give them a task (build a cake, or a castle, or a .....) and 10 minutes to build. Each is on one side and does their build. Then, the parent joins the world and flies about to declare the winner.