Oh good luck with it. My 10 year old son (Yr 6) had the lesson a few weeks ago and I have noticed quite a change in him since. He is not mature for his age and is one of the youngest in the class, and it is my opinion that he is far too young to need to know this information (I am not saying that all children this age are, but my son certainly was).
Since the talk, it is clear that
a) the standard of behaviour in the year group has gone down and there has been lots of name calling of a sexual nature
b) my son, for only the second time in his life, has been in trouble with the headteacher (again, name calling of a sexual nature, which got out of hand)
c) my son has walked around the house with a dictionary in his hand, almost constantly looking up new words that he does not understand. Of those he is prepared to tell me about, the words included gay, lesbian, homosexual, and plenty others he did not want me to know about.
I am not entirely sure if he actually now understands the mechanics of making a baby or of having sexual intercourse, or of the changes that occur in bodies during puberty, which were the aims of the lesson. I am, however, sure that it has opened up a whole new world to him of words, concepts and ways of abusing others that he was not ready for and could well do without.