That's one of the cruelest things a person can do. I could not even begin to think how excited a child is to get dressed up on Halloween and go trick-or-treating, only to have all of the candy they were so excited to get stolen by some punk. If I ever saw someone snatch a child's bag, whether it was my child or someone else, I would chase them down and kick the shit out of them.
However, I could care less if it is done to those greedy kids. The ones who don't even put on a costume but just want free candy. The teenagers who are way too old to trick-or-treat but just want free candy. If you snatch their bags, I could care less.