I have heard of this beforem when I was in school it was kind of a term that vaguely meant kissing because nobody was really sure what it was, hence "Will you sex me?" not "Will you have sex with me". Kids know it is a word they are not supposed to know about.
During Easter hols my sis-in-law was in the kitchen washing up and my six year old neice flicked the TV channel and Jeremy Kyle was on, she went up to the screen and read aloud "Mum, you had sex with my boyfriend. Mum, does that word say SEX? Why does it says SEX?".
Now, I don't know what happened after that but say little one had asked what sex was and SIL had said, "well it's like when grown ups kiss", there would be nothing stopping my neice going into school the following week and asking her beau to sex her.
My God, you don't think they're getting together do you? PinkFizz, she's just not ready to settle down yet......