What if we took the thought of who is paying for the trip out of the equation? Now, what we have is a great-grandmother and great-grandfather who would like to enjoy Disney with their granddaughter and her family. And, this trip will probably be a "one and only."
Now, since great-grandmother has been to Disney before, she "gets it." She understands how big Disney is and how very much there is to see. She is not expecting to be able to experience everything, but think about it. The shows, the parades, the character meals, the rides, seeing characters..... doing even a portion of these things takes time and usually, at least a bit of pre-trip-planning (example, reservations for character meals, fastpasses, etc). And, say what you want about going later in the afternoon, it is a common planning strategy to say-rope droppers fight fewer crowds for the first hour or two of any park opening.
So, a family going to Disney. How could anyone not have a fantastic time? Well, there is one glitch-the granddaughter is not a morning person. Perhaps not even a mid-morning person. And, will she be willing to compromise (since great-grandmother is) and get out of bed and to the parks by late morning? Or, would she be willing to alternate late days with early days for park visitations?
Some people are saying that great-grandmother should either not have any expectations at all-let granddaughter sleep in as long as pleases, get to the parks whenever, OR come up with a variety of back-up plans, such as take the boys and the dad without the granddaughter and meet up with the granddaughter later. I say, what about great-grandmother's wish to watch the joy on the faces of her great-grandsons when they see The Lion King Show, ride Dumbo, watch the parade? Perhaps get a chance to hold her granddaughter's hand as they walk down Main Street and see the castle for the very first time? My belief-it is important to consider everyone's point to view here.
Bete, I think you need to talk to your granddaughter and her husband. The ball is in their court on this one. Without some commitment and compromise on their side (and, they might be very willing to compromise) this could turn into an extremely stressful trip for you. And, I don't think that makes you controlling or anything else. It is just who you are. A rope dropper, a person who enjoys seeing much of what Disney has to offer.
Whatever you decide, best wishes.