Although I would never call someone to complain about them turning my kid away, I can totally understand why the Mom was upset.
Picture your kids, all dressed up, excited, gung-ho to trick-or-treat. At a certain age, this might only be the second or third time they've ever even done it! It is sort of scary to go up to a house and ring the doorbell--scary but thrilling because the people will be happy to see you and give you candy.
Then you come to the house that looks like WILL give you candy. You ring the doorbell, you shout, "Trick or Treat!" And, instead of a chuckle, a "how cute!" and a piece of candy, you get scolded and sent away. This a complete downer, sort of scary but in a negative way and turns you off to the whole experience. The LW basically sh*t on the Halloween experience for these kids.
"You'll shoot your eye out kid!"
The LW should be ashamed of herself.