Has she only been seen by the pediatrician? If so, I would recommend seeing a specialist. There may be something going on that its not immediately apparent. So before buying some, I would definitely seek out further answers. You do not want to mask an underlying problem by just putting a bandaid on it. If, after dr consults, you do end up going that route, I would just order the x small. You would be surprised how adjustable they are.
Btw, dd started in an x-small at 7yrs old after a wrist fracture and I recall them being big on her but at the low level she was in, it didn't matter much. They still worked well. She is 13 now and still in the x-small, though she doesn't use some of the padding anymore. I am sure when she is ready for her next pair (likely soon), she will go to a small.
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