I’ve been having a grand time playing the Minecraft Feed the Beast Ultimate Pack. It’s a ginormous mod pack for Minecraft throwing together some 45–70 mods to extend Minecraft in various ways. It’s terribly complex, occasionally inconsistent, but surprisingly stable, balanced, and fun. I definitely recommend it if you like Minecraft and want to add more stuff to tinker with. I think it’d be particularly good for kids.
There’s a huge number of mods with varying degrees of documentation; part of the fun of FTB is figuring out how stuff works. The unofficial FTB wiki is a good place to start. Some of the big mods I like… BuildCraft and IndustrialCraft add machines, engines, pipes, pumps, all sorts of automation. Thaumcraft adds a beautifully designed magic system. ComputerCraft embeds a Lua scripting engine, letting you write programs for robots that mine and build structures and stuff. And Forestry adds a bunch of agricultural stuff including a crazy apiculture system of bee genetics.
If you want to play it, get the FTB Launcher and use it to install the Ultimate pack. It’s good about installing stuff in its own directory. Unfortunately Java on the Mac is a total mess; you have to set JAVA_HOME to run Java 6 and also configure the launcher to add the XX:PermSize flag when launching the game.