I have to offer a cautionary note here, with the caveat that I don't know you and your DH and family, so I have no idea of your stamina levels. Unless you plan on leaving the party after the fireworks, as many folks do, you are looking at a 13 hr stretch, or longer, in MK. Once upon a time, I would have welcomed that, but in my 60's now, I would not be getting my $ worth out of MVMCP on that schedule.
IF (please notice the big IF) you are all in good shape and have an appetite for that kind of marathon, go for it. Otherwise, you could consider starting your day later, and doing a nice late breakfast elsewhere. Yes, there are places to sit, but an 8 -15 min rest is often not much reward after standing in a line for 20 min to get off your feet.
Or make the deal that if anyone's energy and tolerance is flagging, they are free and even encouraged to hop on Disney's lovely transportation and head back to your resort with no judgement. Remember that pushing to the point of exhaustion in a park before calling it a night still leaves a 30 -40 min commute by the time you reach the bus stop, wait, ride, and trek through your resort to your room. Overdoing things one day can result in vastly reduced enjoyment of the next day(s) for guests of all ages.
I hope your trip is wonderful in every possible way, and there are no wrong decisions here.... only ones that might not work best for you in the moment. Don't be afraid to make any changes that ensure you stay happy and healthy throughout your visit.