In the Junior levels I find vocal soundscapes are a really effective way to get them to begin to think compositionally. Here is an example of a very simple lesson which ends up creating nice little pieces of music.
Using the IWB draw up a series of pictures and then have the children suggest appropriate vocal sounds for them. The children can also suggest what the pictures would be. So we had bird sounds, rustling sea weed, we made body sounds to represent the octopus, screaming for the scary fish, bubble sounds for the school of small fish and a scraping sound for the large rock. Then draw a simple submarine and explain that the sub is on a sound gathering trip under the ocean. When ever he reaches a destination a new sound will emerge. Using your mouse guide the sub around the ocean and let the children make the appropriate sound.
To turn this into a more effective composition add a few percussion instruments that might be appropriate, we addes chimes and marimbas. You can talk about how they are to be played, dynamics, large or small intervals etc. so that each part fits in with the original idea of the composition – under the sea.
Finally record the whole thing in Garageband. Once the recording has been done it is a good time to play it and reflect with the children on how effective the piece of music was and does it need any changes. I actually took this time to teach some of the features of garageband such as cutting and pasting to double up the effectiveness of some sounds and also adding effects. We added some delay into the bubble sounds.
Heres a video of what it might look like.