Thank you for that link. I was going to take some time today to research, and you saved me from that.
I'm not surprised at her actions - she's typically difficult to deal with. But she doesn't have enough power over us to ruin this vacation (or any vacation). I printed the information and we will give it to her next time we are over there. Of course, her reaction will likely be to read only the part that says ..."even if not required", and only actually see the words "not required". Like I said, our next cruise will become a threesome instead of foursome. At least the child is older now, so when we have to explain to him that we can't take him somewhere because of his mother, now he understands instead of blaming us and thinking his dad doesn't care about him.