I've been reading along with this, and with each OP post I still have the same reaction ... this seems to be a recipe for disappointment. I've traveled to Disney with an assortment of people and now I am VERY picky about who I would go with. There have been quite a few that have "asked" to go with me since we have successful trips, but I won't go with them ... I know the result would not be good. I just say to them "tell me when you decide to go and I'll give you hints." Been there done that, including with folks who don't want to go in the morning. What we did with those folks, we went early and then set up a meeting time for lunch. Usually by dinner we split up again.
So many things you have said give me pause ... you are GREAT-Grandparents so older with routine, your DH is 81 with a
scooter, you are talking TWO 5 year old boys who are primarily cared for by Mom - who would want to sleep in and has no "time constraints" and a Dad that has minimal care experience with the boys so either he or the boys might not be comfortable leaving Mom at hotel. Honestly no matter what they say in advance, may not work out on arrival ... their 5 years old. Is it even possible for them to stay up late for a night show, will they willingly take naps, can they handle waiting in lines if you don't do early mornings, can they handle being away from Mom and what will their behaviors be like? Only you know them, and there are so many variables that come into play that you must think about before you even bring it up to them.
It really does not matter how any of us tour, how any of us view our trips as successful .... because we all have different things of importance, we all have different budgets and what results make them acceptable and we all have different levels of expectation. Personally now I want to close every park down at night so if I went with someone who rises early and wants to be at the park by opening .... that would not be a successful trip for me.
What makes a valuable trip for you is all that matters, not what others think.
I do understand the high cost of you paying for this vacation at Disney, and it's not going to be a warm fuzzy memory if you spent all that money and left disappointed.
I feel like the goal should be to spend some quality and fun time with your GreatGrandsons,
building memories so the destination shouldn't matter. I would look at a more low key, less structured and certainly less expensive vacation destination. You need to really do your pros and cons before you even bring it up to them. It won't go over well to offer a trip but if they don't want to work on a good schedule, un-offer it.
Good luck!