I've looked up this stuff before. It's mostly urban legend that the media loves to spread and keep going. Rainbow colored Fentanyl is their latest thing to be afraid of on Halloween. Before that it was Ecstasy and on and on.
This Professor claims that his research found zero legitimate cases of kids hurt or killed by Halloween candy going all the way back to 1958. Says there's always been a twist to the story once investigated, making the claims fraudulent. Mostly by attention seekers and a man that poisoned his own kid's candy so he could blame tainted Halloween candy and collect insurance money.
Still, it never hurts to be cautious, it really could happen.