Uncrustables are prepackaged.
Not all of the counter service restaurants have them on the kids menu and all places also have other items on the kids menu. Most of the table service restaurants don't have peanut butter sandwiches on their kids menus.
We went last fall with a family whose youngest son was allergic to multiple things, including peanuts, tree nuts, fish, shellfish, milk/milk products, eggs.
He has anaphalactic allergies to several of those (I remember seafood/shellfish and peanuts/tree nuts, at least). He ate very well, with no issues during the trip.
Columbia Harbour House would have been able to handle his other allergies, but because they serve so many fish/shellfish items, they said they could not
For counter service restaurants, you tell the CM that you have a person with allergies in your group. They have policies to assist you.
For table service restaurants, it is best to have them note it on your reservation and also mention it when you check in and again to your server. The chef will come to your table and discuss the safe foods with you.
Peanut allergy is probably the most common allergy, so they are well prepared to deal with it.
This is a link to Disney's page about special diet requests:
https://disneyworld.disney.go.com/guest-services/special-dietary-requests/