If kids show up to your door expecting a treat and they have food allergies, be prepared to give them something that's not edible and is still a treat. Glow sticks are big favorites.
If you plan to be prepared to help kids of all stripes have a great Halloween, put out a teal-colored pumpkin. I've seen them at CVS, but they're $8. You may want to tie a teal ribbon on a pumpkin to communicate the same message.
And, sign up at foodallergy.com so that you can add your address to the map so parents know you are ready for their allergic trick-or-treater!
As for me, when our son was a "pup", I would trick-or-treat alongside him costumed as a tired working mother...