As far as I know, the document I posted will do, as long as it notarized.
From the RCL site:
Family Legal Documents:
Adults who are not the parent or Legal Guardian of any minor child traveling with them are required to present the child's valid passport and visa (if required) or the child's birth certificate (original, a notarized copy or a certified copy) and an original notarized letter signed by at least one of the child's parents. The notarized letter from the child's parent must authorize the traveling adult to take the child on the specific cruise and must authorize the traveling adult to supervise the child and permit any medical treatment that must be administered to the child. If a non-parent adult is a Legal Guardian, the adult must present a certified certificate of Guardianship with respect to the child.
Here's the link: