Aaron Carroll amasses the evidence:
We cannot definitively prove that no child has ever been killed by a stranger poisoning their Halloween candy, but we can state that no such event has been documented in the media to the best of our knowledge.
Of note, children do face some real health risks on Halloween, but the risks come from cars, not candy. The National Safety Council reports that children are at an increased risk of injury as pedestrians, being four times more likely to be killed by a car while walking around on Halloween than on any other night of the year. So, if you really want to keep your kids safe, focus on making sure they are visible to cars and are very careful about crossing the street, particularly as darkness falls.