Lori is right, call Carnival to know exactly what they require. However, in most cases of minors traveling abroad without their parents, notarized permission from both parents (regardless of divorce or custody situations) is required. In Mexico that is definitely true. Even if no mention is made of it, I would advise you to have it anyway. You can write this up yourself if you want, or the cruise line may have a sample form to use. You may also want to include having the parents sign permission for you to get medical treatment for the girls should they need it.
You are a brave soul, Tim, traveling with 5 teen girls!