So you will be there for at least part of 8 days. How many days do they have park tickets for? That makes a big difference in how you plan.
I don't think this has to be stressful to plan just because you do have the luxury of time. Not a lot of first timers have a whole week.
Without much guidance from them, my priority would be scheduling at least one day in each park. Then, depending on how many days worth of tickets they and you have bought, add days at MK and Epcot or AK. DHS really doesn't need a second day for anyone these days, in my honest opinion. I would just choose days based on EMH or a crowd calendar (whichever you decide). I think PH would just make planning more difficult for you at this point.
Not sure how old your kids are, but their kids are old enough that I wouldn't plan a whole resort day. If they are willing to pay for tickets for each day, go to a park each day. Everyone is old enough that they can speak up if they are tired and want a rest. Let it be known that the plan is flexible. There are 4 adults total, and someone can stay behind with those wishing to sleep late one day, or some can go back for an afternoon break while others stay.
I would not go to a park on arrival day. You get in late, and that could be later if there are any delays. If you don't want to go to Disney Springs, think about the Boardwalk. Maybe burn through two dining credits at HDDR (it's great family fun), or take a fireworks cruise. Be ready to tackle a park at RD the next day.
Once you have a plan for which park on which day, maybe try to get them in to planning by giving them some choices-- like, on our MK day, we can go to character dining at Crystal Palace or Chef Mickey's. Also, get their input about FPs. Do you live close enough to each other that you could invite them over for some dinner and planning? They are likely overwhelmed by all the choices, but if you narrow it down to a few choices, they might be more interested. I wouldn't give them choices for everything--that would be a logistical nightmare for you. As head planner, definitely make some decisions unilaterally based on what you or your family wants, but something like, "We will try Sunshine Seasons for lunch, but would you rather have dinner in Morocco or Germany? I'll try to get a reservation for the one you prefer." Or "Would you rather we try for a Frozen or Soarin, or Test Track FP for our tier one at Epcot?"