While browsing iTunes the other day, I went to take a look at what is offered in iTunes U and came across some great free recordings of classical music, done by Columbia University. If you want to listen and download for yourself, just search for "Columbia University Orchestra" in the iTunes Store. It should bring up several recordings... click "See All" in the box labeled iTunes U for the free downloads! If you have trouble finding it, let me know and I will post some links, because you really don't want to miss out on these!
While you're there, you might also try searching for "Classics For Kids" to learn about composers, or search for "Little Kids Rock" for free guitar lesson or drum lesson videos (again selecting whatever is in the iTunes U box).