And if she tells you when she grows up that you were mistaken to do so, you accept that correction and move on. Unless you're a total arsehole.
Let's do a thought experiment. Suppose every white person in America decided to say, "I now identify as black!" Would you now go around and say, "Wow, there's no more white people in the U.S.!"
Of course you wouldn't. you would still know they are white but delusional. Would you support people changing their skin color if they feel a different color?
Let's see if you're still saying, "it's no big deal, Half!" People wear blackface and you guys foam at the mouth. "But, it's just a skin color! Why get triggered?"
You do love your false dichotomies, don't you?
There ARE no white people in the US (or indeed, in the world). Every single human has a load of common black ancestors, and every non-albino has some melanin in their skin. The amount varies in a continuum from a little to a lot; At no point is there a clear line, with everyone on one side of the line obviously and indisputably 'white', while everyone on the other side is obviously and indisputably 'black'.
The idea that there are two completely and indisputably distinct human genders, into one or other of which every human being can be placed by the examination of a universally agreed and universally applicable definition, is utterly absurd.
No such definition exists. If you try to define what makes someone a 'man' and not a 'woman', or vice-versa, you will discover that there are always exceptions - people who either fit into both categories, or into neither.