I should mention this in rebuttal to people complaining about the controls. When I started playing Terraria (on PC), I had only just started learning from Minecraft how to properly move in a video game with the WASD keys, and I picked up Terraria's movement and menu system fairly quickly. If I can do it with little background in 'action' games, I think an eleven-year-old can pick it up easily enough.
With that aside, I just wanted to say that games like Terraria, Minecraft, and many other games were not intended for kids. The developers did not design these games with kids in mind. They designed them to be fun. You have to keep this in mind when looking at video games for your kids. This doesn't mean that kids can't play these games but, well, this is like letting your kids watch The Animaniacs; questionable content may pop up where you don't expect it.