Forgive my ignorance...

There may be other ways, but this should work...

1) Make a camcorder video of your son playing, while recording into GarageBand.
2) Export your GarageBand Project to an audio format like MP3 or AAC.
3) Import your video into iMovie
4) Drag your music track into iMovie (you can do this from the Finder or you can import the music track into iTunes and then drag the music track in from the Music Browser.
5) Drop the music track onto the video clip from the camcorder.
6) Line up the audio clip by dragging it so it exactly matches the video.
7) Select the video clip and open the inspector. Set the volume to zero.

If you want a quick Tutorial on how to handle audio in iMovie, [here it is|http://www.apple.com/findouthow/movies/imovie08.html#tutorial=audioclips]. The tutorial is for imovie 08, but the techniques still work in 09.

If you save the iMovie with a video preview, you can access it in the media library FROM WITHIN GARAGEBAND.

Just drag it to the arrange area - GB will create a video track and separate out the video sound, so that you can then mute it or mix/fade/whatever it with additional audio material.

GarageBand is far better at dealing with the audio track for a movie than iMovie is.

This is by design.