My child is about 6 years old and has had 3 days of high fever, plus he is complaining of calf pain and being hard to walk.?

Muscle inflammation. The process you describe is producing an inflammatory reaction in the kids calf muscles with fever. There are a number of possibilities that will require an urgent evaluation to sort out. It could be something simple or something bad.
SEE YOUR DOCTOR. Most often it is a good practice to have a child seen by their pediatrician after 24 hours of fever. Go see your doctor!
Take him to MD ASAP. This child should be seen by a physician soon. Will not try to entertain a diagnosis on such little history and without examining him, but he sounds very ill and the conditions i can think of will require professional intervention. Hope he recovers soon. Give us a follow-up.