Here's my top tip for parents of kids with allergies, like gluten intolerance or nut allergy. When they're old enough to understand the first Superman movie (but not old enough to know any astrophysics), tell them that you found them in a field, just like Superman. You discovered later that they came from an alien planet that was destroyed in an explosion. This planet orbited a star made of gluten, peanuts or whatever (a star made of bee stings might be pushing it a little), that's why it makes them sick, just like kryptonite makes Superman sick.
They'll want to know about their super powers, I have no advice for you here, it's going to vary from kid to kid, be creative.
Emphasise that it's a secret (or they'll probably get a kicking at school) and see how long you can keep this up.