I noted the allergy when I made my reservation online, then a week or so before we left, I emailed Special Diets (
specialdiets@disneyworld.com) with our ADR Confirmation Numbers and a specific description of his allergy.
They responded very promptly (same day) with detailed information about how allergies are handled at WDW. A PDF of "tree nut friendly" foods offered at the various park and resort eateries was included, but they did stress that we should speak with a chef / manager at each location for other available options. (This was great advice because the PDF implied he would have limited options, which was not the case.) They did ask me to send more information about our dessert party reservation (data, time, party size, person with allergy and his needs), which seemed a bit repetitive, but better safe than sorry.
When we checked in to Launch Bay, the CM asked who the child with the allergy was and then kindly suggested our first priority should be to speak to the chef who was located by the frozen Nutella station. (My nephew is actually not allergic to hazelnut, and eats Nutella, so going over to that station wasn't an issue for him.)
The Chef walked my brother-in-law and nephew (aged 7) through the regular offering stations, while another CM went backstage to get a bag full of individually packaged treats that were all tree nut free, included ingredient lists, and were kid friendly. The bag included a marshmallow treat, a nut free brownie (a rare treat my nephew particularly enjoyed), some ginger cookies and some other things as well. There were a lot of things in the bag and he was also able to eat many of the regular offerings (the pistachio dessert and the hummus were definitely off limits though). He had so many options that most of the allergy-friendly treats were kept and consumed over the next few days.