This is not my area of expertise, so I caanot offer you advice beyond suggesting an appointment with her doctor. He/she may refer to a rheumatologist or podiatrist. Be sure to have this checked out, though.
There are a variety of rheumatologic diseases that cause swollen joints and can affect children (e.g., juvenile rheumatoid arthritis, ankylosing spondylitis, etc.). Also, some digestive diseases (celiac disease, for instance) can cause joint swelling. I recommend a visit to the pediatrician.