People do murder each other's children from time to time. This does happen.

If you want to murder someone's children, and those children are known for taking all the candy in a "take 1" honor-system bowl, then poisoning candy might be a means to your end.

I expect the odds of being poisoned on Halloween are less than 1/1000 the odds of being run over by a car.

Watch for cars and maybe hang glow sticks on your kids, or give the a flashlight, or use reflective tape or something.