Classicsforkids - learn about music. Compose and play games.
Piano Explorer - Piano Explorer magazine online
eMusicTheory - practice with interactive drills.
Musiccards.net - practice with flash cards.
Scramble Saurus - unscramble words about music.
Virtual Piano Action - explore how a piano key actually works.
BBC Musical Mysteries Tour - musical mysteries games.
Blank Sheet Music - a great resource for any musician.
Piano Learning Center - piano learning center of the piano technicians guild
Chicago Symphony Orchestra - info about Chicago Symphony Orchestra concerts, most especially their family matinee series and special concerts for kids.