I’m similar to your kid. I have celiac and also severe lactose intolerance. Fortunately, a lot of candy is free of most allergens. For example, Sour Patch kids and Laffy Taffy. (Not sure if it’s vegan though.) I also had a nephew with a peanut allergy, so I try to read labels when I buy Halloween candy (though this year I decided to reverse trick or treat the houses on my block with kids and then turn the lights off, since kids don’t really go out anymore like they used to.)