I took my mil (82 at the time) last Sept and she loved it! She's planning on doing the FL-CA repositioning cruise with my hubby and I in May! While we aren't taking any children, we get the benefit of seeing other people's kids having fun, but when they're having a meltdown, we can walk away!!
In Dec '97, we did a family trip (just to WDW, not a cruise) and my folks got a separate room. It was great if one of the kids got too rowdy and needed a timeout. Or if they had school work to do. Also, my parents got to spend some time with each kid (we had 5 total) during the course of 2 weeks.
I always let my mil decide what she wanted to do. I'd tell her, I want to do this, are you up for it or would you like to do something else? Sometimes she did what I wanted, other times she solo'ed.
She is a diabetic, and after a few days of someone else cooking for her, and food being available 24/7, she was really "firing on all cylinders" if you get my drift.....my sil was amazed when we got home, since my mil was more animated than she's been for quite a while. The stimulation was great for her!!
Good luck, Kelli