It looks like it is effecting her right foot more—is that correct? An injury might be possible. Riboflavin (vitamin B2) deficiency usually causes curled under toes, all of the toes normally. Sometimes Mareks disease can mimic riboflavin deficiency with curled toe paralysis in one leg. I am not sure that splinting the foot would help at her age when bones are fully formed, but might be worth a try if it is an injury.