Paedophile's Halloween

Listen up, you're the kind of person who thinks a participation trophy is a real achievement. A paedophile is a disgusting adult who is attracted to children. "Free delivery" in this context is a dark joke because it refers to how kids are often alone when trick-or-treating on Halloween, and are therefore vulnerable to being targeted by a bad person. The joke is using that dark reality to try and get a laugh from people who like dark humor.