Last year when my daughter was 11 she was experiencing leg pain to.  Her doctor had put her on a high does of dex and took her off of prednizone because her levels were out of wack which I believe was way too much.  After she started taking the dex she gained 20lbs and complained that her legs hurt all the time.  She could hardly run in soccer and baseball.  I researched the drug and found out that one of the side effects is leg and joint pain.  I then had the doctor put her back on the pred and she is fine.  She lost weight and all the pain stopped.