No need to worry about this pigmentation and consider this as beauty spot. This sounds like a birth mark which does not affect the sight. If you want to make sure about her sight, cover one of her eyes at a time and ask her to pick something very small like sweets or mustard from the palm of your hand. If she succeeds, her sight is good. A good eye specialist should be able to dilate her pupils with eye drops and check whether she needs glasses and also check the back of her eye for any abnormality.