Hilary,
My kids are a little older now (elementary and middle school), so I am not up on the new music expressly for kids, but I'll tell you what I used to buy for my kids...some of which my younger child still listens to:
***classical music, particularly Mozart, but also Handel and Tzaichovsky...Nutcracker, etc. We would take our kids every year to the Ambler Orchestra's children's concert at Wissahickon High School, and sometimes to Philly Orchestra concerts, and we always bought in advance whatever pieces were going to be performed at the concerts. My kids still listen to their classical favorites.
***international and ethnic music, not only from our own background, but from other backgrounds. My youngest still likes to go to sleep to music in European languages that none of us understand, but which I picked up on some business trips to Europe! (Some are children's CDs, some are not, but I always ask an English speaker who works in the store about the albums, to make sure I'm buying appropriate music...even if we don't understand the language!)
***folk music...the music from the 1960s is so classic...and our kids have grown up listening to Peter, Paul and Mary, among many other artists
***music my husband and I love. Our kids have grown up listening to Billy Joel, Simon & Garfunkle, the Beach Boys, Carly Simon, and many other artists from the past who transcend to today.
***Broadway show tunes. Make sure they are appropriate, though! (There are some great shows with great music that may have a line or two that you wouldn't want your child going around singing!)
***There are some great kids' CDs where kids sing popular tunes.... I can't remember the name of any of those, but there's a whole section of those CDs at places that sell CDs. Our kids have numerous ones of those and love them. "Sugar Beets" was the first one of those albums we received, for our daughter's 1st birthday, and our kids still listen to it!
***Some popular artists for adults have put out music great for kids. For instance, our kids LOVE a certain Michael Finestein CD...I forget the name of it...but it has great songs for kids on it, although it may not be marketed that way. My daughter, 13, still listens to that music often! I think we owned the CD even before we had kids.
***Likewise, long before we had kids, my husband bought me a CD where popular artists sing Disney, etc. I think it is caled something "About the Mouse." Our kids still listent to this!
Have fun! Our kids are extremely musical, and I have no doubt that part of that is that we exposed them early and often to all kinds of music!
--T