Have you tried the new Music Memo app from Apple? It's very simple and definitely a first version, but wow does it have possibility! My third graders and I actually used it. Here was our process:
1. Select an elemental melody form (example: a b b a)
2. Select a melodic pattern for each section: a = mi re do, b = mi so mi so
3. Sing it, make changes as needed. Then add lyrics. (listen to one class' composition below).
4. Record the song using Music Memo. Music Memo struggled to analyze what key we were in based on our simple melodies, so I had to quickly edit it to reflect that we were in C. From then, we added the bass by clicking on the bass picture. Then we added drums by clicking the drum picture. Then I simply shared it to Soundcloud! We did all that in a 30 minute class period (granted they were already quite familiar with elemental forms and melodic patterns).
5. In following lessons, we wrote a word chain to create a drum B section and we are currently working on playing these melodies on Orff instruments. We will record it again once it's finished and polished, and we might even add in some recorders.
PROS to MUSIC MEMO: It's incredibly easy to use. It's a no-brainer and the fact that it analyzes your music so quickly is fantastic. This is a great tool for composers, and it was a great chance for my students to go through the composition process the exact same way a "real" composer does. Also, I can drop my Music Memo file (any version of it I generate) into Garageband to add loops or other live instruments. The tempo settings even transfer.
CONS: Because it's so simple, it's really just a good starting point. This is not meant to be a final recording app, so it's just not. This is great for keeping great ideas that you build on later. Because of this, the analyzer isn't perfect and it can require a good deal of editing if you want the drums to play along perfectly. Or.... just export the track into Logic or Garageband and finish it there!
Try it out - it's a handly little free app, and I love that Apple thought of us music-making folk.