It was always called the "fanny" at our house. And I still call it that today. I'm from a Polish family and the word "dupa" was used only in slang talk.
My father always called it tuchus which he mispronounced by making the first vowel a long O sound. I think that he liked to talk to me about things like getting shots in my tuchus. Still today I start to get hard when I hear the word.
In our house it was the yiddish word tuches too. The closest to English pronunciation is "took-us", and it means buttocks in yiddish. When I hear it now it brings back some feelings of embarrassment, but not excitement. In terms of RT's and suppositories, even when we were very young my folks would not have said "we need to put this in your tuches", they would have used the word rectum. You might get spanked on your tuches, but you had your temperature taken in your rectum. I have read that "tuches" might be the word that the word "tush" was derived from.
When we were very young we were told to call our butt a hiney poop was called grump and my dick was a tinker.
I usually used the word " Butt ". My Dad used either the word: " Bee--hind " or " Bottom " when my childhood misbehavior reached critical mass and my (bare) BeeHind or (bare) Bottom was nearing or about to get a "tanning".