Mint or Elementary make sense.
Another idea - if there aren't any real requirements from the system other than allowing your kid to poke around - try something like Haiku. It's not Linux but it's interesting and different and is still Unix-like.
Another idea - if there aren't any real requirements from the system other than allowing your kid to poke around - try something like Haiku. It's not Linux but it's interesting and different and is still Unix-like.