awwwwwww (hugs) I have no experience in these type of discussions - but do you have to include Santa in the discussion as well? If not, maybe take strength in that time that she found out about him.
I don't know how to deal with that, but I think if I was faced with it - I would just say it as it is, gently and if asked questions say something like you dont learn this stuff until you turn 11???????
OOOOOHHHHHH - Maybe, ask the school for notes on what will be discussed on Tuesday and then on Monday night, go through and share them with your girl, prior to the info day. Then, she can ask any questions to you and then also she will be be 'informed' on the day, and then you can laugh and brush aside any form of being embarassed etc .......
Might just open up a line of communication, that she will trust with you forever ......
Good Luck, as I said I have no idea, got no kids - but if you know what they will be teaching, in the way that they will be - it might just make it easier to teach your kid yourself and break the myths, and the reasons why.
Love Send
and
WOW - that is a HOOLEYDOOLEY situation!