We took my late MIL (then aged 78, but pretty physically fit) when DD was 9 mos and DS was 10yo; she finally caved to DS' pleading. She later told DH that "I am never traveling anywhere with that slave driver ever again!". (The slavedriver being me.) So, what did I do, you ask? Well, I planned which park we would go to each morning and each evening, with a midday rest break in the middle. We also planned a no-parks day mid-week. We stayed in adjoining rooms at the Swan, which has the least amount of walking I could finagle. At no time did we try to make rope drop, but we did close a park most nights, in order to do most of our park time at night when it was cooler. We were fine with her not coming with us to parks if she didn't feel up to it, but she didn't want to spend any time on her own without the kids. Because we went in late September, we knew it would rain. We warned her to bring appropriate footwear for rain because we don't leave parks for rain showers, but she apparently didn't believe that, because she was astonished when I just passed out ponchos and kept walking.
So, what was the problem, other than the rain? Mostly Fastpasses. She just could not wrap her mind around the idea of making an appointment for a ride, especially since she didn't much like the rides, and it drove her crazy to have to cross the parks without stopping in order to make FP times. She loved the shows, but otherwise mostly just wanted to sit on a bench & snuggle the baby (who was on her 3rd trip, and of course was a character junkie even at that age, she kept crying to follow every character she saw.) MIL also felt very strongly that the baby shouldn't go on any rides, so she tried to sit them all out with the baby, which frustrated the baby, who didn't want us to leave her behind, AND disappointed the 10 yo, who wanted Grandma to see at least some of his favorite rides.
MIL had offered to watch the kids one night so that we could go downstairs for a nice dinner, but that didn't happen; she bailed on it because she was tired and realized that they would get antsy in a hotel room. We did suggest she take them to the pool, but she was afraid to, even though there were plenty of lifeguards and DS swam like a fish at the time. (We were not really disappointed by that episode at all, we just canceled the dinner and took the kids back to a park. I only tell that part of the story to illustrate that a plan to have grandparents babysit might not work out.)
I should also point out that I got along fine with my MIL, though our relationship was a rather politely formal one; we were never best buddies because our interests were very different. My mistake on that trip was in not truly understanding that despite her saying that she "wished" she'd had more money and opportunity to travel, she actually hated travel. She didn't like being away from home, and really wanted us to stay with her and keep her company at all times, which, since she didn't like rides, was kind of problematic at WDW with 2 active little kids who were experienced with Disney World and eager to do their favorite activities. In hindsight a few years later, we realized that she had probably begun to develop minor dementia at the time of the trip; so she wasn't just uncomfortable in a strange place, but probably frightened as well, but at the time she just presented as a tad grumpy.