Hi Penny,
What a fabulous experience for your granddaughters: a stay at the
Polynesian Village Resort with their grandmother!
Your picky eaters are in luck. In recent years, Walt Disney World has made a concerted effort to expand healthy food options for all guests, especially children. Cartons of Greek yogurt normally are available at resort food courts, such as Capt. Cook's at the Polynesian, and in gift shops that carry food items.
And you should be able to find it for your granddaughters at a number of quick service restaurants throughout the parks under the dessert or snack section of the menu. For example, it's on the menu at several restaurants in Magic Kingdom, including
Columbia Harbour House and
Cosmic Ray's Starlight Cafe. It's also available at
Harambe Fruit Market in Animal Kingdom.
I don't know of specific locations that carry Greek yogurt in the other parks, but I suspect you will find it there as well.
Please note that you will have a small refrigerator in your room at the Polynesian, so you'll have a place to store a supply if you want to bring it with you or order it delivered from one of the grocery delivery services that serves Walt Disney World.
Thanks for visiting the Disney Parks Moms Panel. Wishing you a magical vacation with your family!
Wilma