It's difficult when you look for btdts and don't find them even here! I can't offer one, but I think my instinct would be to talk a lot about what being a woman means to me, and do some anticipating: "when you grow up, I wonder whether you'll want to...". I'd acknowledge how mysterious gender identity is, and how strong it can be; I might mention that, very occasionally, someone who has a boy's body complete with penis etc. feels they're a girl, and how lucky she is that she'll get the kind of body she wants just by waiting! I'd definitely talk about the function of childhood and how important it is to the human species that our young get looked after and get to learn for so long before they grow up.
