I'd create my lessons using the following goals:
This last idea is important, because it is a huge jump. Luckily there are a lot of examples to draw from. The biggest one is Peter and the Wolf. But that's a little too big, so I use Weird Al Yankovich's "Peter and the Wolf," because on that album there are several animals that have been transformed into music. And I use the Jaws theme song as an example as well. With these examples, kids get a real clear picture in their mind on how to emulate an animal with music, and Garage Band helps them not worry about many of the technical issues that goes with composing music.
This 5th grader created a Garage Band song that duelled a springbok with a lion:
This all leads up to the final piece- turning one of the song titles from the "Music of our Lives" CD into an actual song. It's another level of complexity, and I'm sure it would have mixed results, but how cool would it be to actually think about and compose a piece of music that tells the story of a song title that is about you? Again I can turn to Peter and the Wolf, but I could also turn towards Wynton Marsalis and his Blue Interlude (The Bittersweet Saga of Sugar Cane And Sweetie Pie).
Only the cover art is slightly inappropriate
I used to listen to this album in high school, but it was only really when I started thinking about teaching music that I realized how great it was and what it was trying to accomplish. I think this would be a great musical journey for upper elementary children. I wish I could teach music.