In the same way that pulling a child's pants down and even being touched there at the Dr's or being administered a suppository isn't causing trauma. Children don't have that inherent understanding of "private area" that you claim they have. A lot of things can be uncomfortable but that doesn't mean that they're traumatic on a pathological level. (For what it's worth, there are people for whom getting their hair cut just a tad too short feels traumatic -- doesn't mean it actually is)How is pulling a child’s pants down and slapping an other wise private area NOT causing trauma?
You keep viewing this through the lense of an adult that's grown up in a society that stigmatizes body parts as well as your own personal trauma.