Hi, Susan! A whirl-wind one day in Walt Disney World! OK, I am going to be totally honest here. If you only have ONE day, I really would try to pick just one park since you have 2 little ones in your group. I recommend one quality day where you can enjoy all there is to offer.
I would take that day and spend it in the Magic Kingdom. Have a double stroller so both your grandchildren can be resting while traveling between attractions, and can even catch a little nap when needed.
Plan all the major superstar rides for little ones: Peter Pan, it's a small world, Many Adventures of Winnie the Pooh, and for your 5 year old: Haunted Mansion, Tomorrowland Speedway, and even Big Thunder Mountain if up for some adventure! For some "down-time" the family can enjoy Tomorrowland Transit Authority, Hall of Presidents, and Carousel of Progress.
Schedule a buffet lunch at the Crystal Palace where you can enjoy a special lunch with Pooh and his pals. Make sure to make Advance Dining Reservations at (407)WDW-DINE or
online. I suggest having a quick breakfast in your room, scheduling an early lunch at Crystal Palace, then grab quick service for dinner (Cosmic Rays is a favorite for my family.)
Believe me, with all there is to do in the Magic Kingdom, your one day will be filled! Enjoy Epcot when you have more time and your grandchildren are a bit older. If you are determined to do both parks in one day,
contact me and I will work out an itinerary for you.
I hope you and your family enjoy your one VERY magical day in Walt Disney World!