Almost everybody has heard of the song Wheels on the Bus. I suspect there are at least a few dozen variations of the song. With this app your son can sing along with some of the traditional verses. For each verse this application presents a beautiful illustrated scene. Each of these scenes has items that your child can interact with. My daughter particularly liked the monkey stealing the cupcakes.
One of the best feature of the app is that you can change how the song is played. There is an option to switch the signing to Spanish, French, German (and others) and even making it just instrumental. There is also an option to record your own rendition of the song.
The Duck Duck Moose screenshot is very fun and creative, and we had to replay that scene over and over again.
As for the bad: I wish there were more interactive items on each page, and that it included some extra scenes even if they were not part of the original song. My daughter played with it for over an hour (with me), but I think in the end she was tired of the same scenes. For smaller kids this might not be a problem.