Why are kids supposed to say "Trick or Treat"?

I found out why kids are supposed to say "trick or treat" when going door to door on Halloween. Here's the story.

I took Norah (2) and Dejaquan (4) out around the neighborhood this time. Norah was wearing fairy wings and Dejaquan was a storm trooper. At the beginning of the candy trek, I would came up with them to the door and tell them to say "trick or treat" and "thank you", as they obviously are too young to understand the process without some help. Dejaquan took off his storm trooper mask after just one house because he said it made his face sweaty. After about 5 houses, they were just getting the hang of saying trick or treat when the person would come to the door.

We rarely allow my 4 year old, Dejaquan, have candy or gum, and he actually likes gum more than any candy. So when he got a piece of gum from a house we had stopped at, he thought he would start asking, "Do you have any bubble gum?" after ringing the door bell. So as he started asking that question instead of trick or treat, I thought it was humorous and let him ask it because it showed his interest in the whole thing and the candy givers were getting a kick out of it.

"Do you have any bubble gum?" Dejaquan would ask.

"Sorry, no, I don't have any gum." the person would reply.

Door after door, he would say it religiously. But something happened that made me realize what he was doing was inappropriate. Norah started saying it too. Ugh, I was kicking myself for having let it go on so long that she thought that's what was supposed to be said. So I had a talk with them about how it was rude to ask for something specific and we should be grateful for whatever they might have to give (one woman was giving out handfuls of pennies). I explained it was the same as when responding to what they give you with "hey, I already have this!", which Dejaquan managed to say a couple times. That one I jumped all over.

By the end, they were saying trick or treat, and thank you. Even though it was the second time I had gone trick or treating with my kids, I still learned a lot from the experience just like them.