There are several reasons that may cause eye pain in children. The following conditions may cause eye pain:
Conjunctiva. This is the most common eye disorder happen to both children and adults. It will make eyes look red. But they also come with the symptoms of itchiness and discharge.
Foreign bodies. If there is foreign bodies into her eyes, she may also feel pain. If she have rubbed her eyes, it will appear red. If so, you shall get the foreign bodies out of her eyes.
Stye. If your she got stye, it is also painful. So, you'd better take her to doctor to get her eye checked.