We don't really get any trick-or-treaters in my neighborhood but if we did, I'd be printing out some of these little flyers to add to the bags. What a great way to let kids and parents know that your goodies are compassionate!
There are two versions which you can download here - a Vegan only and a Vegan/Fair Trade option. And remember watch out for carmine (insects used in red candies) and those with gelatin (ick!).
Here's a list of compassionate candy, but always double check the ingredients.