It's the same scare that's been around since he olden days. Seriously. They used to blast this in the news in the 90's before we went out Trick or Treating.
The original is a boomer humour comic complaining about PM10 pollution from cars and all the allergies it gives kids. The message of the comic is that we should invest in public transit and walkability to reduce car pollution.
As with all things - the poison is in the dose (and delivery). It would depend how much fentanyl is added to how much candy, and how much candy the kid eats. Also how old and how large the kid is.