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.
The app isn't sophisticated in terms of giving you a lot of control. That's not what it is for, but I suspect it will keep young people entertained as they try to come up with some worthwhile "compositions."
The app looks like it is set up to offer some more options soon, like additional moods or orchestrations, which will probably be available via an in-app purchase. As it is, this is a clever app that should engage the youngsters in your life.
The app is optimized for the iPhone 5 and requires iOS 6 or later.