We did finally take a cruise for dh's special birthday. It was a 4 nt on RC with friends. Couldn't do a
Disney cruise w/o kids! We also flew down day before so really 5 nts total.
They were 11 and 8 at the time. Older one was fine and understood that it's important for parents to stay married and still love each other, etc. He said, "You're right mom. You deserve it." Little ds cried when I 1st told him. Tried to break it to him gently and when the timing was good. ie, not at bedtime or before school. He was 'just ok' with it knowing it would be my parents coming to stay at our house and their whole routine would be like any other day except that on the weekends, Mom mom & Pop pop would do some fun things with you too.
By the end, my parents were exhausted and we joke that my mom burned rubber backing out of our driveway.

My kids stay overnight with them often at their shore house so all is used to that but never have they done the daily grind of morning routine, homework, dinner, karate, etc.
Little ds had some hard times during school and teacher sent a note home saying, "We are sooo glad you're back!" Nothing awful specifically but I guess it made it harder for him to focus.
Overall, I felt really bad for little ds and we agreed to not do it again for a long time. Since then, my mom has retired but also taken in my grandmom (her MIL) since she's almost 93 and cannot live independently anymore. My mom has her hands full and I wouldn't ask more of her now so that I can go on vac w/ dh. She still takes them for 1-2 weekends per summer to the shore so we do still get some down time alone; quiet, empty house just not a real 'going away' vacation.
btw, the cruise w/friends was sooo awesome. That I don't regret.

They want to make it an annual thing but I just can't do it.