San Francisco Symphony's SFS Kids
If you're looking for a fun introduction to classical music, check out the SFSKids site. It has music to listen to, information on major composers, and fun little tools for learning about important musical concepts like rhythm, tempo, and pitch. It's also a great place to learn about orchestral instruments.
The "Radio" is a good place to start. The Radio offers six stations, all playing short classical pieces. As each piece plays, accompanying text describes the theme of the piece.
With some pieces, the text includes a link for playing the piece yourself on the "Performalator." The Performalator is an eight-key keyboard, with each key a different color. Music notes above it are color-coded to match the keys to be played, so by following the colors and clicking the mouse on the keys, kids can play a short phrase from the music.
Kids that are interested in the sounds different instruments make will love the "Instruments of the Orchestra" section. They can see photographs of each instrument, hear its sound, and read about its history and function in the orchestra.