Sounds like the one about the Canadian with the two assholes.

There was a Canadian guy, Johnny, who died in a workplace accident on a job site. His face was basically burned off by a molten metal spill and when the coroner was investigating he needed help identifying the body. He went to Johnny's co-workers and asked if they could identify him, everyone pointed to Mike, an American, and said they were good friends so they should ask him to do the identification. Mike is brought to the body, sees there's no face so he flips the body over, yanks down its pants. The coroner is freaked out, but Mike looks up and says, "nope, not Johnny. Johnny had two assholes."

The coroner is totally taken aback and just says, "uh, okay, I'm going to have to ask someone else, is there anyone else who knew him well?"

Mike says, "we'll, you can ask Joe, the other American. The three of us went out for beers every night after work."

So the coroner grabs Joe, and takes him to the body. Joe looks down, sees there's no face and then does exactly what Mike did and goes for the butt. He looks up at the coroner and says, "nope, I'm afraid this isn't Johnny. Johnny had two assholes."

The coroner is baffled and decides this time he needs to ask for clarification. "Mike said the same thing, and you have to know that sounds crazy. How could he have two assholes and how could you even know that!?"

"Well," Joe responds, "every night after work, me, Johnny and Mike would go out for beers. Every night, when we got to the bar the bartender would yell out, 'Hey, here comes the Canuck with the two assholes.'"

Someone negative is now out of the picture: That ass is gone! "Mommy, what's ass?" Another word for butt.

I know Mom has a butt, because everyone has butts, but she said "that butt is gone", meaning she must have had a second at some point. Concusion: Buttectomy must have been performed, which is an oupatient procedure, thankfully.

If this kid has any younger siblings, maybe they overheard a joke between the parents while mom was pregnant. Something about two heads/butts and four arms/legs.

I knew a woman who had two young (pre-teen) daughters. For whatever reason, she taught them to use the term "front butt" for their vaginas.

Maybe this kid's mother had a hysterectomy and explained it as losing the front butt (although I know that's not correct, it may have been a simple explanation for a small child).

What's so confusing? She had a single buttectomy.

Im going with a speech impediment mispronounciation of "bird".

Maybe she had an absolutely insanely gigantic butt, the size of two butts from a child's perspective, and she got liposuction so now she has one normal-sized butt.

Sounds like a mistelling of "I was born with a second asshole. But my twin sister doesn't like when I call her that."

"I was born with q second asshole, but then my twin brother died."

Spuds most likely. She ate one.