WHen my daughter was 5 she had a golf ball sized tumor removed from her back side. It was about 2 inches below her tailbone, on the inside edge of her "crack" on one cheek. Not the greatest spot when one has to explain it to anyone who needed to know why my child could not participate in activities for a very long time.
For months it hurt, was diagnosed as an unknown tumor/cyst, needed watching, then finally removal. But during all of that time, she could not actively play as it had unknown origin, so the chance of it rupturing could not be taken, so no falling down, etc etc. I had to explain this to teachers and substitutes, school people, other Dr's, etc. And so did daughter. Then there was the long healing process, as the tumor was removed and had to heal from the inside out, meaning she had this LARGE hole in her, not sewn up, etc. She had to now, in addition to all of the previous months of caution, now had to sit "funny", had a pillow to sit on at school, special rules, etc.
We made a game out of, coming up with all the silly names we had ever heard a backside referred to. For a 5 year old it was pretty fun and a silly way to make light of what was going on. Most have been mentioned, but honestly, I do home childcare and I do avoid the word "butt". I tend to say Tushy or Booty or Bottom.
But we had people sending my daughter cards and little gifts with all these words! Bum, booty, tukus, boo boo, hoo hoo, "the bottom of your top", backside, seat, tushy...