Hi Elizabeth!
Abby is 2, so she doesn't qualify for the Disney Dining Plan. It's for children ages 3 and up, so this means that you'll either pay out of pocket for her meals (where it is not buffet or all you care to eat), or she can share from your plate.
With her history of multiple allergies, I' make sure to email
specialdiets@disneyworld.com with any concerns prior to your vacation. Also note in ANY table service meal reservations that there are food allergies, and they will work with you in either choosing something that is allergen-free (and preparing it in a way that prevents cross-contamination), OR making something from scratch to suit her needs.
At Counter Service locations, prior to placing your order, make sure you let the staff know there is an allergy, and the manager will work with you to find something safe for her.
Have a magical vacation!
Amanda