Windosill is one of my favorite online puzzle games. It's quiet and wonderfully strange and comes with no instructions.
Patrick, the creator of Windosill has just released an iOS app for kids called Metamorphabet, which contains the same whimsical quirkiness. It's not a game, instead, it's an app to teach kids the alphabet. Each time you touch a letter, it morphs into something different that starts with that letter. I just wish there were 1000 letters in the alphabet.