If I can give perspective from the other side. I am "the DIL" obviously not the original poster's DIL lol but I am a mom of kids with ADHD, sensory issues, and one who is immature for age. It's often NOT easier for grandparents to handle it without the parents. You are not with them daily, so you don't see the daily quirks and redirection and what works and what doesn't on the mundane daily tasks. So I would prepare yourself as much in advance to learn those things, what works, what doesn't, what to do in the middle of a meltdown and what not to do. We have well meaning grandparents who are absolutely not allowed to take our kids on a trip without us because we've experienced one to many times where they don't know what they're doing and make things a lot worse and then the kids are miserable. It sounds like you don't want DIL to go. I am in the exact place of your DIL with the in-laws not wanting me to be there. But usually mom (and dad) are the ones who know how to redirect. So again, it would help you a lot to have DIL fill you in on what works and what doesn't. Otherwise you're going to have miserable kids in the middle of meltdowns. I'm a local and my own child with ADHD can't last more than a few hours in a park and would prefer to spend the entire time in the pool. Just another perspective coming from care and a mom who has traveled with grandparents who have a lot of ideas about how things will go and then are frustrated when the kids are acting up.
While you're a seasoned Disney visitor, I would let go of any expectations. The kids might want to swim in the pool the entire time. They might want to spend a lot of time playing in the fountains in Fantasyland instead of riding rides. Let them do it. Let them stop and don't worry so much about how much they can see or do. It's tough to say what they'll want but odds are that it might not be what you think. We went on a trip with grandparents years ago and my husband's dad reminded my husband how much they had paid for park tickets and the kids were done after about 2 hours and just wanted to swim in the pool and hang out in the hotel room because the hotel room was more exciting to them than rides. lol.
It's great to hear that you're not park commandos. I'd let them sleep in or at least sleep until their normal wake up time and not rope drop unless they actually normally wake up that early. Make a decent breakfast with protein. If you're rushing, they're going to feel stressed.