We did a 3 generation trip in 2016, with my parents (70s, physically fit, but my mom has dementia), my sister and SIL and their daughter (infant), DH and my 2 girls (then 4 yrs and an infant). It was easier in some ways because my parents had been to WDW many times when we were kids, so they knew the drill, and my sister and I each had babies so we were all in the stroller set. I was the primary planner and linked everyones tickets, etc. My sister's family was coming from CA so add in jet lag...
I got great advice from the board (and there's great advice here!), the main points of which are:
- Do not try to do everything together as a group.
- Plan for at least 1 meal, or 1 meal a day, together as a group.
- Be understanding/respectful of others' schedules and needs - including spending habits.
- Understand different people's vacation styles and try to find common aspects, rather than trying to fit subgroup A's style into subgroup B's.
While we were in the planning stage, my dad was very much like, "just tell me and mom where we need to go and when." My sister was like, "I like to relax on vacation! We don't need to go to the parks, we can just hang out at the pool. And we like to sleep in." We were all staying in a 2BR at BLT.
They ended up coming for the middle 5 days of an 8 day stay, which was fine. I suggested certain things for them to do, but told them up front that since we'd pay a penalty for no-shows on an ADR, and they weren't willing to commit, I'd book most ADRs for my family and my parents, and if they wanted to try to add on later, we could do that, but no promises. They sort of but didn't really keep their baby on CA time, and my parents were on CA time as well, so the 4 of us in our immediate family would hit rope drop at the park of the day (there was at least that planning) and we'd text to meet up around lunch time, which was morning to them. My dad and mom got up early a couple times with us to hit the parks early, because they decided they'd rather do that and get a nap midday when the kids were napping. On the whole, we tended to see each other around early afternoon, either in a park or at the condo (BLT), and then later in the evening in the condo after the kids had gone to bed.
For FPs, I made FPs for my parents and sister and SIL in the late morning or early afternoons, knowing they'd be late risers, while ours tended to be in the AM or we'd have a few overlapping ones. That actually worked pretty well, since we were able to use rider switch to ride some of the coasters together while the grandparents watched the kids. Often if we met up in a park, it was for a casual (no ADR) lunch and 1-2 rides, and then we'd be heading back for naps with our kids while the people on CA time toured the parks some more. If we had dinner ADRs, they were for around 5pm, and mostly just with my parents. We'd either meet up back at the condo or hand off with my sister, and I think my sister and her little family would either get or make something casual at the condo or in the parks, while we took our parents to dinner. For us, the meals are part of the fun; for my sister's family, they were a refueling stop.
My parents took us (my sister and I) to WDW a lot when we were kids, but they were always very frugal trips, so I really enjoyed treating them to staying at BLT and dinner at CRT (things that were a thing when we were kids), among other things. They enjoyed WDW with their grandkids. My sister's family did WDW their way, which wouldn't have been my way, but it was good to reconnect with them as well. (They ordered groceries on their 2nd day, and then the next day made it to AK at like 2pm because they got up late and were making sandwiches... <facepalm>)
As others have said, it seems like OP does not have kids, and the kids will dictate a lot of the schedule, or at least dictate the availability of the kids and one parent. A hangry or tired kid can make things unpleasant for everyone. Just expect things to take longer than expected, and be open to surprises and new experiences. As a PP has said, Enchanted Tales with Belle is really fun with little kids, but if there are no kids in your group, eh. Our 2yo loved the Tiki room as well, and the carousel and Small World are always popular. We have in the past left 1-2 adults with the toddler and kids to ride the carousel over and over again while others were using FPs to go on other things, and then switched off later.