In reading about this grand plan of theirs, it just seems more like they want the bragging rights of being able to say that they took all of their grandkids to a cabin for a week. "Yes, Marge, it was wonderful! We had ALL of the grandchildren there!" "Oh, Jane, you are just such a lucky woman to have all of your grandchildren around you like that! I envy you so much!"
WOW! That is one hell of an interesting assumption!
There is NO way any of us have evidence that warrants this sort of negative judgment about the grandparents.
I have to admit this assumption about the grandparents occurred to me, too. It is
uncharitable. But, in my experience, when I have had people tell me they want "all my grandkids" or "all my cousins" or "all my co-workers" to be present at something, it's not because they want me personally, as an individual, but because they want to have everyone in the group together at the same time, to all be treated the same way. Like Grandma saying she wants all her children and grandchildren to come to someone's wedding, so they can have a family picture taken there--it's more about the appropriate warm body being there for the photo, with little to no concern about whether that individual can or wants to attend the event. I have heard people have, or repeat, the exact conversation GrammarNerd imagines. Again, that is just my (unfortunate) experience.
If DD has plenty of other opportunity to see her grandparents, and the cousins who would also be at this gathering, I don't know why the grandparents would be so pushy/disappointed that she couldn't attend this particular
gathering, unless they were placing disproportionate emphasis on the "all" part.
Anyway, the OP has offered to bring her DD to the gathering for a long weekend--did the grandparents accept that offer or not? That should be perfectly sufficient to fulfill their wishes of having "all" the grandchildren there at the same time, for two or three days.
If it were my child, I would also be concerned about just the grandparents, with no other adults, looking after all the grandchildren. I would want to know (well, I suppose I would
know) how many kids that would be, what the spread of ages was, if any of them had special needs, general health/competence of the grandparents, etc..