Slapping their hand away for a boy or a girl will only bring about the feeling their is something wrong and it's taboo and dirty. That could lead to problems in adulthood. There simply is nothing taboo or dirty about one's own body. It's the vehicle that gets us all through our own path in life and is to be treated as best and kept in as best condition as we possibly can.
Thats what i wanted to say to - just couldnt get the words out. I grew up in abit of a taboo household, and i believe that it has affected me as an adult.