The Piano Wizard company has long been selling products to help teach kids (and adults) how to play piano and read music. They carry applications for your computer that you run along with a MIDI keyboard (your existing one, or you can buy one from them). There are multiple steps to teach you what you need to know.
Step one starts with a game akin to Rock Band, and it progresses all the way to reading regular musical notation. By breaking it down into smaller steps, new musicians aren’t as intimidated, and they aren’t being presented with as much information at once.
Now Piano Wizard has a Kickstarter campaign to develop a version of their program for the iPad. They must have a pretty good start on it so far, since they expect to deliver it in November. They’re approximately halfway to their goal of $20,000, and still have a month to go. Once they reach this minimum, they plan to offer stretch goals as well.
If you or your kids are interested in learning to play piano or read music, give this campaign a look. I’ve backed it, and plan to use it along with my kids.