I am a music teacher, so this is "up my alley". I highly recommend this resource:
www.musick8.com/
They put out magazines and CDs every two month that music teachers subscribe to. But they also aggregate songs into CDs around specific themes that are sold individually. There are also tons of free resources there for music teachers. Their songs are so appealing to all ages (including middle age like me, LOL!) and they have some that I love to use for character education. For example, one of my favorites is "Motivated" and I love "Growing Up" (means doing things you don't wanta do sometimes). I don't think you can download single songs, but you can listen to excerpts and get the CD that has those songs or whatever, I think.
Another thing they offer is a site just for kids. There are usually some freebies (for KIDS ONLY-not kosher for teachers to get them) on there and parents can put an "allowance" there for a child to download songs he/she chooses for 99 cents each. I like that it teaches budgeting as well as music.
Here is the kids' site:
www.musick8kids.com/
Diane Woodward
"Cadenza"