Disney Springs can be a nightmare on the bus so I would suggest a taxi for them to get around.
If the grands did go into a park, do you know which is most likely? I think you have great options at any of the monorail resorts, or Boardwalk resorts for things to do outside the parks. If they enjoy people watching, sipping a drink or snack, you can't go wrong with any of those.
The monorail resorts give them Wishes and the Electrical Water Pageant every night by the water, the GF has a band in the lobby every day.
Boardwalk has so many snack, drink, and people watching options, they won't get bored. The boat goes between all of the resorts, and there's Epcot World Showcase for a quick visit. At Boardwalk and Beach Club, there are reasonably priced
DVC options, if you want a combination of rooms and villas. GF villas and Poly bungalows are much higher priced. If Grandma enjoys cooking, a villacould be a great option for all of you too if she prepared some meals while you were at parks.
Give yourself space, you don't need to be next door to each other so don't stress about that.
Just my 2 cents off the top of my head.