I was lucky to be able to bring my children up in a place where they did not have religion forced on them in any way. (Yes, I know some people think RE lessons are not that -- but not even that "Some people believe ..." schtick , if that makes sense.) They did get, from parents and teachers, enough history and literature and so on to enable an amount of cultural appreciation of religious themes etc. And they grew up, I reckon, decent, moral, thoughtful people.
OK, some of my children now live in England and have children themselves. These (my English grandchildren) talk to me about, amongst other things, their schoolfriends, their beliefs, and so on. Most of them all but one, in fact see religion much as do their parents: as faintly absurd but still not to be laughed at in the face of believers because, well, politeness.
One grandchild, however, has had the thought, "Well, it could be true."
My response? -- "Yes, perhaps, but there are some things that cannot, could not, be true: like if someone believes that a circle could be square, for instance. That would be silly, wouldn't it?" So, I suggest this grandchild look at these beliefs in that light. " Could that be true ...?" Many religious beliefs, in fact, could not be true.
The idea that someone could be 'born in the wrong body' falls in this latter group. It is not just that some people think it or believe it, whereas some others do not. Rather (and this goes for men being women, or a boy changing into a girl, etc., too), it could not be true. Of course, the 'politeness' point applies even in such cases (just like 'immortal souls' and so on). But still, children can see the point here, I find.
So there, if you like, is a response to a child who is faced with transgender ideology: "We shouldn't laugh so they can hear us that would be unkind but it is really silly , isn't it?"