We do allow the kids to have ice cream from BK - my gluten/egg allergic child is a plain jane, his preference will always be for plain vanilla. My nut allergic child loves the chocolate sundae.
BK is a special treat in the house because it repeats on DH and I so bad. We power through for DS1 every once in a while because it's the one place he can have fries in all of the places we can eat out as a family.