I agree that the penetration of the touchscreen interface will grow in the future, but I can't understand why you would want to limit your potential universe to just the Apple universe when it is, and is likely to remain, relatively tiny.
You can already do some amazing things with books in Flash, and Flash developers are a pretty common species as 98% of browsers in the world can run Flash (only around 1-2% of total browsing is on Apple mobile devices). So creating a very cool, interactive children's book that can run as an app or within a browser is relatively easy and cheap to do. And once it's done, it can be made available to around fifty times more people. So lower costs and higher income.
And if you're concerned that the iPad will be the only tablet touchscreen device available, apparently there's somewhere between 50 and 80 devices due to be launched over the next year (and none of them will be able to run iPad apps)