Honestly, I look at it like this. If the child is interested in learning and wants to learn and excels at their studies, then a few days out of school isn't going to matter in the long term scheme of things one way or the other.
If the child is disengaged from school, uninterested in learning, doesn't do well then a few days staying in school instead of the cruise isn't going to suddenly put the spark of education in there.
There is a grey area between those two for kids who are interested in school, do okay but that a few days out can have an honestly detrimental effect. Obviously this partly depends on WHAT few days are out - for example right before an exam.
Every child is different. Only the child's parents having an honest discussion with the teachers (not the grandparents) would know where their children fall in these lines.
PERSONALLY, My son is in that middle grey area. He likes to learn and wants to do well in school, but he fights daily against his Aspergers. For him, missing school can be detrimental, so I make the choice not to take him out.
If the children the OP is discussing fall in either of the 2 main categories above, it makes no dang difference if they pull them out for a couple of days. If the parents and teachers approve, I don't see why any of us should have an opinion - those two groups know best where the children fall.