If your child still has open growth plates, typically 16-18 and younger, you need to come in for an x-ray and a full exam right away.  In children, the weakest point is the growth plates; in adults it is the ligaments. Therefore, it is very rare for a child to "sprain their ankle".   Typically it is a growth plate injury or fracture.  It must be fully evaluated and properly treated to avoid further damage and potential growth disruption. 

Dr. Misty McNeill
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D.P.M. - Founder of Prairie Path Foot and Ankle Clinic