Don’t ask them for input. They don’t know or seem to care. My guess is they are more concerned with spending time together than seeing all that Disney has to offer. You plan your days (keeping in mind what they can do and how much time they need to rest) and then just lead them along. You might have to say “we are getting a fastpass for this ride. Is that something you would like to do or would you rather sit and get a snack or people watch while we ride?” I would be sure to mix in lots of low key activities through the day so great grandma isn’t sitting on a bench alone for two or three hours straight.