When I was about your daughter's age, there was a promotion in my comic where you sent off for some sort of gift. I can't remember what the item was but there must have been different versions for boys and girls because the application form had a space labelled 'Sex'. Never having seen the word before, I innocently asked my mother what 'sex' meant. She was well into some sort of fumbled explanation (which was totally incomprehensible to me) before she thought to ask where I'd come across the word! :-)
Chris