Great question w/pros & cons. Ultimately it will b up to you & ur spouse. You know best...usually lol.
Do you think they could put the offer off for a yr? Would that be a compromise worth looking at?

I do understand separating the kids for a week away & enjoying them w/o their sibling. I also think it's sometimes easier for the grandparents depending on their age too. It would be a nice break for you & your spouse too.
Going by age, I think the 6yo would be fine, but if your next one only turned 5...that one might be hard. Can the 6 yo do a week & the 5 yo do a few days like a long weekend?

I also understand how a grandparent says they do things their way. Mine said they "have the right" to bec it comes with the title. So ice cream at 11am was ok I guess. But if they believe in spanking, I'd have to draw the line to how you want your kids disciplined. Pick your Wars & stand by them. Also give them the tools they would need as an alternative to spanking. Something that's acceptable to you & ur spouse.

It'll be hard for you to see them go if you choose this, but it's also a growing experience for them as children. They might have a blast. They might not even remember it either. Just don't know.
Wish you luck in your decision.