Hi Tim,
There were some changes made to the Disney Dining Plan last year in order to make it more flexible and better value for Guests.
Effective May 31, 2015 at Quick-Service restaurants and
select merchandise locations, Guests may substitute one Quick-Service meal entitlement for up to 3 eligible Snack items within the same transaction.
My fellow Moms Panelist Leslie did some great research on this very topic last year which you can read
HERE. Whilst visiting the Walt Disney World Resort shortly after the changes were made, she tried to exchange 1 Quick-Service meal entitlement for 3 Snack items at various locations and discovered that not all locations were participating in the snack exchange. She also found an article on the Orlando Theme Park News website which listed all of the locations which were participating in the snack exchange at that time. (You can find the article by doing a Google search for "Orlando Parks News Snack Credit".)
It is of course possible that changes have been made to the participating locations since this article was published and therefore my best advice to you would be to simply ask a cashier at any location where you would like to do the exchange, if it is possible or not.
From my own personal experience, I can tell you that my family have been able to exchange 1 Quick-Service meal entitlement for 3 Snack items at various Disney Resort hotel food courts and Goofy's Candy Company at Disney Springs during previous visits.
Wishing you a magical trip filled with memories which will last a lifetime!
Emma