Always, always tell about food allergies. That isn't something to make light of and those who may laugh have probably never seen an allergic reaction or someone going into Anaphylactic shock (depending upon the severity of the allergy.)
Cross contamination is an easy thing to have happen in a kitchen and
especially in buffet areas. Even if the kitchen staff don't cross contaminate on the buffet, the patrons very well might. How often do you see people using serving utensils, etc., in multiple dishes?
Disney is great about allergies. Tell them about the allergy when you make ADR's and they will make a note of it on your ressie. Typically, the chef will then speak to you when you get to the restaurant, but if they don't, just ask.
If it happens they don't serve kiwis, then great, once you find out there are no kiwis on site, then you'll relax and have a much better experience. And, if they do, then great again, you'll be informed and be taking the care you need to avoid them. Win-win, either way and no big deal to have taken the time to make sure.
Have a wonderful trip!