What Causes Pain In Joints In The Teenagers?

I am the mother of 2 teenage children, a 16 year old female and an 18 year old male, who both complain of joint pain on a regular basis. The pain occurs primarily in unilaterally in the knees and ankles. They both have suffered multiple soft tissue injuries in their ankles but no knee injuries. My daughter weighs about 185lbs at 5 6 and my son about 135lbs at 5 10 . I m concerned about these complaints because they have been ongoing for many years. Both have had xrays that are negative but no further diagnostic testing has been ordered by their physicians. Suggestions?