as someone whose parents wanted to go to disney for their 80th birthday celebrations, i agree with master yoda. there is nothing they cannot do.
some advice:
if you can afford to rent a car, and your grandparents can walk short distances i recommend it. made life easier. that way we only had to deal with the ecv in the parks.
my parents used a disney rented ecv all day at the park, that way you can just go the parks without having to transport 2 ecvs back and forth from the park in either the bus/boat/personal car. if you are staying onsite there are free wheelchairs at the bell stand to use to help them from the car to their rooms. the bellmen held them for us while we were gone and brought them right out to the car at the hotel entrance when we returned (then someone went and parked while we pushed them to the room).
at the parks, we would park in handicapped spots and walk short distance to where you rent the ecv. at some parks there are complimentary wheelchairs to get from parking lot to park entrance.
downside to this plan- a few minutes spent renting them in the morning when you arrive, it is not as cheap as renting from an outside company.
upside- transport as above, and if it breaks down (or your mother runs into something and it breaks- just saying
) or the battery dies, they drive a new one to you instantly. never waited more than 5 minutes for a replacement.
as far as going onto/into attractions, the CMs have this down. at the entrance, they always ask if you can "transfer". yours can apparently. so you have choice of parking them where the strollers park and walking the short distance to the ride- but be warned some queues are LONG- or riding right up to the front and getting off there. the CM at each ride will give you directions for that ride if they choose not to park at the strollers. oftentimes, we would approach the CM and ask which would be easier at that attraction and they will advise you. the CMs seem to go out of their way to help anyone on an ECV, always helpful, pleasant, almost embarrassingly kind to my parents.
remember, slightly older folks may not want to do tower of terror and expedition everest, be sure to explain each attraction to them ahead of time and make your plan accordingly. although my dad did insist on doing big thunder and splash mountain! 80 years old- what is wrong with him??!!
based on the fact that your grandparents are probably not super-young, plan a trip home for a rest every afternoon. we did. many days they wanted to stay later than we planned because they were doing so well, but we went home for at least a little while, despite what they wanted to do, and they did fall asleep every afternoon when we went home.
don't push them too hard, i learned this early, you don't need to do every ride for your grandparents to have a wonderful time. sometimes less is more enjoyable.
sit down meals in air conditionin gave my parents a chance to rest and recuperate while in the parks. do at least one a day