OK if I understand this right, they are going to be there for a month. In that time frame they have plenty of time to go to the parks while staying off property. A four bedroom villa in Kissimmee will be a blessing for that amount of time, especially for free. Driving to the parks is no big deal when you are there that long.
For me, I would take two days at a deluxe, one at WL, and later, one at BC.
During those two resort days I would not go to the parks at all, but enjoy the resort and Disney bubble from some of the best Disney has to offer. WL is spectacular, the lobby, grounds, pools, restaurants are all special and a little time there is wonderful, and they can take the boat over to all the monorail resorts and do some resort hopping and even watch Wishes from the Poly beach.
Then BC...that way they can experience a different resort area, and enjoy the best, biggest pool at all of Disney, Stormalong Bay. And they can also resort hop there and see all the Epcot area resorts and eat at Beaches and Cream.
With a month there, there will be plenty of time for the parks...I would recommend SeaWorld as well.
So doing a couple of deluxe resorts, and using those days for resort time, they could experience wonderful resorts. I would not recommend this approach if they plan on just using the resorts for sleeping. That would seem like a big waste of money and time.
With a month there, they already have the time issue covered.
Now my suggestion is based on the assumption that they will be going to the parks plenty of other times while staying off site. If the only time they will spend in the parks is while staying on property, the whole issue changes. Then I would recommend value or moderate to maximize park time with the grand kids, but again, if there for a month, there will be plenty of park time. Heck, buy annual passes and go for a few hours ever stinking day if there for a month...that is what I would do, except for the couple of days when I was resting at WL and BC.