My son asked this a while ago.
I said that mummies and daddies make babies together, with a seed from dad and an egg from mum.

That satisfied him.
Except a little later he came back to ask why he'd never SEEN us making a baby.....
awkward smile.... Ahhhh..... That's special mummy and daddy time.



Moi - it's great to be prepared for the questions, but she probably won't ask for as much detail as you think at this age

Good luck with your discussion, I'd be curious to see how you go . Of course she may have forgotten all about it and may not even be interested anymore