The Four Seasons by Antonio Vivaldi are great pieces for children (and adults). They are also wonderful to have around for when you are hosting a party for background music!
"Baby Genious" & "Classical Music for People Who Hate Classical Music" both feature pieces from the The 4 Seasons ("Spring" to be specific I believe).
But I recommend getting the whole set, we have it from the "Masters of Classical Music" set (volume 7), performed by various artists. This CD also has a few other of his most popular pieces on it as well.
But your best bet is to listen to music you like , and then pass it down. Play soothing during naptime, and a little more rambunctious during playtime, and somewhere in-between for creativity (the more instruments, with less crash, boom! for creativiy that is) <img src="/images/graemlins/grin.gif" alt="" />
Let your ear be the judge!