If you have a lawyer, perhaps the lawyer could draw up a legal doc. stating everything you said above. If not, I would at least type something up and have it notorized.
Some countries have a problem with children being bought and sold, which creates the need for extra documentation. I have heard that Mexico is particular about this sort of thing.
Have you tried calling the cruise company and asking for a Supervisor? They should be able to tell you what is necessary.