Your child's general provider should be able to provide guidance initially.  Fibromyalgia can be difficult to treat but often can be helped by a physiatrist (specializing in children with chronic disorders), a rheumatologist, or an alternative medicine physician with other tools available (including hypnosis, relaxation techniques, etc).  Chronic use of pain killers is often avoided, and if necessary, an expert in pain management can assist in finding substitutes and weaning off the chronic medications.