http://www.bbc.co.uk/science/humanbody/
Interactive site with games and quizzes. Has animated sequences of muscle movement.

http://www.kidshealth.org/kid/body/muscles_noSW.html

http://www.kidshealth.org/kid/stay_healthy/fit/work_it_out.html
Why Exercise Is Cool: Explains why exercise is good for muscles.

http://yucky.discovery.com/flash/body/pg000123.html
Facts about muscles.

http://www.sciencenewsforkids.org/articles/20040505/Feature1.asp
Feature article about warming-up and stretching muscles before sport. Is it really good for you?