Sir Sampleton - Your iPhone Can Sing Like A Casio SK-1
I don't usually sling around the word “genius”, but I'm pretty sure that Paul Slocum is one. You might remember his name from the brilliant Atari cartridge he created called Synthcart. Pop this in your 2600 and your 70's game console becomes a playable analog synth.
His newest project is Sir Sampleton, a project to bring back the fun of sampling random audio to play on a mini keyboard. Paul is an old friend, so I can speak with confidence about his love for the old Casio SK-1. His Sir Sampleton app has that fun lo-fi Casio spirit, and yet it fits in your pocket. It's a very unadorned interface, but the simplicity belies how much cool stuff you can do with it.
A child will like monkeying with it, just as much as a musician will find it gratifying. At only $3US, it's also quite a bit cheaper than a real Casio these days. Just search for “Sampleton” in the App Store.