Here's an iPhone app that's great for kids just learning about music. Dr. Smith's Music Factory, (free) listens as you hum or whistle a tune and then composes music around it. You can also touch icons if you don't want to rely on your ability to carry a tune.
Here's how it works. You select a mood. So far, Sweet and Funny are supported. You hum, whistle, tap on the screen or randomly shake the phone to create notes. After a few seconds, the song is composed. If you save it, you can then share it with friends or family via mail, MMS, Facebook or Twitter.