Minecraft is a popular app used by many children. It is a game based on finding resources, crafting, building and combat (if you choose to). Children can explore different maps and build different worlds. 

Children can play the app version in single player mode which doesn't allow them contact with other players.  They can join together with friends on the same wifi network.

The online version allows multiplayer games with anyone online. It is safest to play on a private server that only friends can join. Children under 13 need parental permission to play the online version.