Speaking from the grandparent side here....and as a veteran of numerous multi-generational trips!
DH and I appreciate that our stamina is waning as that of our DGSs is increasing and we cannot hope to keep up with them at every step. We have always stuck to the idea that there is NO need to stick together at all times, and that if our energy is flagging we are responsible for managing our own breaks, or sitting out an outing. After a week of touring in last September's oppressive heat, I rented a ECV for the
MNSSHP. It made a huge difference and worth every penny that night.
If your parents are not easily able to manage their own downtime, you will have to make slightly more detailed accommodations. By this, I mean if they are not able to opt to leave a park and travel back to your resort for a break, or do not like the idea of spending time on their own in the parks. This was the case when we took my 89 yr old mother to WDW for the first time. We headed to the parks each day just past rope drop, and usually stayed until 2 or so, then headed back to our vacation house. We'd enjoy our pool, have a home-cooked meal, and head out again right after dinner. Worked well for the young ones, too. Often Mom would opt to pass on the evening outing. She was able to drive a
scooter that trip, and walked a good deal, too. Two years later, at DLR, we used a regular wheelchair, but she loves DLR, and could still walk when she felt like it. We would take her back to the resort mid-day, and scoop her up again at dinner time. We stayed onsite there, and it was very convenient to be able to walk to the parks!
we are looking forward to our Nov trip with our grandsons, and this time DH has opted to only get a 2-day pass and
MVMCP ticket. He wants some real down time on this trip and is also going to try to find some tennis players in the area. We love vacation rentals, because we enjoy having lots of space, as well as being under one roof. This time, we will get 2 cars so we can go in separate directions as desired. And he can always upgrade his pass if he finds he is missing out on the fun!
I hope you have a super magical trip and make loads of wonderful memories for everyone. Oh, we got the Memory Maker on our last trip...and LOVED it!