Music Editors - a great range and for those who don't use a Mac and garageband - some real alternatives!
Myna Audio Editor
Others worth looking at are:
Soundation
Jamstudio
Looplabs
Other Music Sites
The Science of Music
This is a fantastic site that lets children investigate different sounds and make their own mixes.
Other sites worth looking at:
The New York Philharmonic Kidzone
DSO Kids
