Minecraft is a fun Java game by Markus Persson, in where you build blocks and tools, and in some modes fights "mobs."
There are multiplayer and single player modes, so the single-player ones are child-friendly, but with multiplayer, the chat can turn to "some topics"...
Minecraft is a game about placing blocks to build anything you can imagine. At night monsters come out, make sure to build a shelter before that happens.