Hi Paula!
This sounds like a great way to spend a vacation with ones grandparent - can I come too? I promise to act as a personal tour guide and be an extra pair of hands to hold backpacks and cameras. If not, I do have some ideas to help with your party's potty problem.
~The majority of restroom entrance locations are within eyesight of each, meaning you shouldn't have to send your grandson that way while your granddaughter goes this way, and they both become out of sight. There are also usually benches or gift shops nearby that you could park them on or in while you take your break.
~Study this
Restroom map for all Companion locations. These single-potty bathrooms are meant to assist all kinds of guests, and can benefit your group in that if you all go one-at-a-time, you can ensure that each child is safe when it's their turn.
~Aim for the Restrooms near Quick Service Dining Locations as you can actually knock out a few pit stops at once if you play your cards right. Locations like Sunshine Seasons and The Electric Umbrella, both in Epcot, Restaurantosaurus and Satu'li Canteen in Disney's Animal Kingdom, ABC Commissary in Disney's Hollywood Studios, and Columbia Harbor House and Cosmic Ray's Starlight Cafe in Magic Kingdom Park are all indoors, so you can take a break from the weather, as well as grab a meal or a snack. Have them stand in the food line while you make a quick trip, or leave them at a table snacking while you take your break.
~Another option would be a Baby Care Center. You can leave them just inside the entrance while you make a quick trip to the ladies room around the corner.
~And be sure to download the My Disney Experience Mobile App as there is a convenient interactive Restroom locator.
I hope this helped, Paula, and We'll See You Real Soon!
~Kristen