Wait...Amazon App Store? So you didn't put it in Google's Play Store, too? There are roughly 60x more users with access to the Play store than Amazon Appstore. I don't know what you have heard about Amazon's app store - probably that is more "efficient" at making money "per app" than Google's Play Store, but Amazon has a much smaller user base. We're basically comparing 350 million users (Android) to 5 million users (Amazon).
Also, if you do port it on the "real" Android and the Play Store, you should try an ad-supported free version, too. The problem with paid apps on Android is that Google doesn't have a very large Checkout/Wallet user base, and it's a huge obstacle for people who don't have this account to make it right then for a single app that they may want. They are building the userbase now with ICS, since they are asking everyone to sign-up for it, but ICS only has a 7% market share so far, so it will take a while until everyone has Android 4+.
It's also worth noting that if his app is relevant to non-US customers, there are more than a few regional app stores worth looking at. Not to mention the Nook Store (B&N had better children's books than Amazon for quite some time, so that might be an important market for a kids app).
Also, if you do port it on the "real" Android and the Play Store, you should try an ad-supported free version, too. The problem with paid apps on Android is that Google doesn't have a very large Checkout/Wallet user base, and it's a huge obstacle for people who don't have this account to make it right then for a single app that they may want. They are building the userbase now with ICS, since they are asking everyone to sign-up for it, but ICS only has a 7% market share so far, so it will take a while until everyone has Android 4+.