I am going to join in on this one, dont do it, dont allow it, no way, no how ... unless you are going to go with your child. too many bad things can go wrong. I am surprised that these kids parents would encourage them to go. Natalie Holloways mom thought her daughter was with responsible adults and look what happened.
Carnival has the policy that all cabins must have a 25 yr old in them for a reason. I would strongly discourage this trip.