I have a DD age 2 who is allergic to peanuts and I have to carry epi pens for her. We've been to WDW 3 time with her since we found out about her allergy and we've never been asked to show documention for her allergy and they are always packed in the diaper bag in case she has a reaction on the plane. I do though have a note from her allergist saying that she has a peanut allergy and that she needs to have them with her at all times, but like I said no one has ever asked to see it, and we fly with her a lot! Good luck!
Diane