To ensure the safety of any minor travelling, most cruise lines ask any adult taking responsibility for a minor to have the correct legal documentation to show they are permitted to take the minor on the cruise. They may also ask for the document to show you can sign waivers and other such documents for the child.
In the case where an adult is taking a minor and they are not the Parent or Legal Guardian, they will request documentation to show the adult has the permission from a Parent/Legal Guardian to do so.
Each cruise line may have different rules around these situations. We advise you to check with the cruise line's website at the earliest opportunity so you have time to get the necessary documents.