They should carry a copy of her birth certificate, a copy of any insurance card she might have, a letter from her parents allowing the uncle and grandmother to travel with her and a letter from the parents allowing the uncle and grandmother to authorize any needed medical treatment for the minor child. Contact numbers for the parents should be included in the letters. If the neice has a school ID (my 12 yo does) she should bring that as well, it is not a legal ID but could be useful to prove she is the child in the letters.
btw: these suggestions are based on my parents traveling with my children, not any official information. They were required to have the letter to transport the boys into Canada, they would have been turned back at the border without it (agent at border told them that.)