Allergies Québec offers a nice way to include children with allergies in the Halloween party: participate in the My Turquoise Pumpkin initiative, which began in 2014 in the United States and which is now being rolled out in around fifteen countries.
First, you get yourself some non-food treats: tattoos, pencils, erasers, bouncy balls, stickers, etc.
And to make this known to children with allergies, you have two options: you paint your pumpkin turquoise, or you put the initiative poster on your front door. They will then be able to reap surprises in complete safety.