I have read your question,Osgood-Schlatter Disease
(vern.knobby knees) generally shows a sign of outgrowth around the knee area/below the patella (=knee cap) , well as you didn't mention any such condition in the question and without physical medical check up its is difficult to come to a conclusion.
Anyway i Appreciate your interest in doing research of you own.
Treatment for O-S D is generally Rest, Ice, Compression, and Elevation, and if required acetaminophen
or stronger if in 'acute phase' & (the pain is severe and continuous in nature). The condition usually resolves in a few months, with a study of young athletes revealing a requirement of complete training cessation for 1 week (on average) and gradual resumption of full training by 1 month.
Often the Patients suffering with O-S D Don't show any swelling in the ankle region ,
Since you have said that you child is into sports one of the possible reasons could be Tear of meniscus
, or sprain in the legs.
In Such a condition i recommend you to consult a general physician and then a physiotherapist For further treatment.
Wishing Your Child a Good Health and Bright future,
Dr.G.Gayatri Devi (General Physician)