Wil, if your daughter one day tells you she has the brain of a boy. You use male pronouns, or, conversely, you ask him what he wants to be called. Usually, someone transitioning chooses a new name from the opposite gender, and you use that as his new name.
That's easy peasy.
You do what these parents did.
Your child is a human being, even if you lack unconditional love, even if you can't get over your freak-out because you're not okay with trans people, you treat your child with kindness and dignity because that's what they deserve.