Minecraft, at least on Raspberry Pi, has a full API. It's likely a bit complex to get going with, but you might find you can build a layer your kids would find "useful".
I've been thinking about that, especially as my son keeps trying to do things that the game doesn't support out of the box.
He set up an "honor system" store where people were expected to asynchronously trade (by dropping) some commodity for some other commodity and got upset at people "stealing".
Either this generation of kids will be the best citizens the world has ever seen, or the worst.
http://www.stuffaboutcode.com/2013/04/minecraft-pi-edition-a...