Dallas Symphony Orchestra Kids a fun guide to instruments, orchestras and composers. 

Sheet music 

toplayalong.com - free sheet music with backing tracks to play along for recorder, violin, flute, trumpet, alto sax and clarinet. It consists of free music of different styles and levels which can be downloaded and printed off if necessary. Each piece has 3 audio files; full music, just the piano accompaniment to play along, and a slow version to play along. The speed can be increased or decreased. 

8notes.com - this site caters for a large range of instruments. Traditional, world, rock and pop, film and improvising possibilities. Activities for each piece include piano accompaniment with and without instrument/facility to change key of piece/change tempo.