I won't claim to have any expertese with children.
This is one of those situations where you will essentially, increase the child's awareness of it's environment. Since the mother has agreed to it, that makes it the better. But your objective here, especially with such a young child, will be to give them the information they can manage, help them to over come any negatve notions they may have been introduced to and reinforce the normal boundaries that the child will need to learn.
I would emphasis the happiness aspects. That some people, very few, but some, find they are not happy with what they were born with.
For more intelegent and mature children, I might ask her how much she likes being as girl. Then ask her how she might feel, waking up to discover that she looks like a boy.
If she askes about anything personal, I would say to her that just as you have parts of your own life that are personal, so does everyone else.