Mom was the primary caretaker in our house. She always called it my bottom. All of my older sisters when they took care of me also always called it my bottom. I am guessing that they learned this from mom, since she taught all of them how to take care of me.
Mom's neighborhood lady friends that would also sometimes take care of me (if mom and all my sisters were out of town), also called it my bottom. They also called their bottom when referring to their own children.
At the clinic where I went to for all my physical checkups and sickness visits, all the nurses always called it my bottom. The doctors at the clinic sometimes might say my backside but most of the time referred to it as my bottom.
I had heard other children in my neighborhood and at school call it backside, bottom, butt, fanny, rearend, or tail.
In school one of my teachers had asked anyone to do a voluntary research paper on the words for backside, bottom, butt, fanny, rearend, and tail. I remember there were several boys and girls that brought in information they found during their research. When the teacher asked if anyone did any research, there were several that raised their hand. It was voluntary to go up front and read out loud to the classroom on the information that you found. There was one boy and one girl that in their research had discovered the word posterior. The teacher thought that was the best suited word to call your bottom.
To this day, I still call it my bottom. This is when talking to doctors, nurses or any other healthcare provider.