I would go along with those who advocate telling children the truth about this (and lots of other things, btw).
It is impossible for people to change sex. If you are a boy, when you grow up you will be a man; if you are a girl, you will grow up to be a woman. If you were a girl when you were born and you grow up, you will be a woman when you die. Hopefully an old woman, but, yes, a woman. No sex change possible.
Just the same as when/if children, as they do, come across stories about other strange beliefs. Some people think they were somebody else in a past life; interesting story perhaps, but, no not even possibly true.
Gruffalo, good story, useful moral; but actually not true.
Rama rescuing Sita from 10-headed demon King Ravana with the help of Hanuman and his monkey army; fine story, good festival associated with it, maybe helps us think/do good things, but not actually true.
What about people who think they lived as an Egyptian pharaoh? Or tell you that 10-headed demons are real ?Well, there are many ways we need to be kind to other people, even if they do believe strange things. None of those ways involve us accepting silly things as true when we know they are not. Changing sex is one of those silly things some people think they believe.
(Father Christmas is a tricky one sometimes, of course. I suppose we have strategies in place for him, though. And it would be strange in the extreme to connect belief in Father Christmas with belief in the possibility of changing sex. No genuine conflict here, in other words.)