Quoting [.The Tiger Mistress*.]:" I personally don't like to take chances with children. there may be a few that don't prey on children, ... [snip!] ... an open buffet during times of the year like Halloween. Being a mother, I firmly think that its a very solid grounded law. "
If they want a kid on Halloween, they aren't going to be stupid enough to snatch one that came to their door. Too many witnesses. Most trick or treaters travel in groups, or with their parents. Not easy targets.
If they want one, they'll get one another way.
Keeping children safe is about keeping them in groups, keeping them in well-lit areas, teaching them HOW to be safe, and monitoring them.