The child of your grand niece (also called great niece) is your great grand niece or nephew (or great great niece or nephew). If you are a woman, you are that child's great great aunt; if you are a man you are the child's great great uncle.
The child of your niece is your great nephew (if the child is a boy) or your great niece( if the child is a girl). You are the child's great aunt, because you are a woman. A man would be the child's great uncle.