DifficultPifcultLemonDifficultcan I ask, how did your DC decide that they were trans instead of a tomboy girl, or a boy who...I'm not sure how to word it....was 'girly'? Why didn't they just wish to be who they were and have the interests that they have, regardless of if those behaviours and interests were traditionally thought of as for girls or boys? And how did they come to the conclusion that they wanted to identify as the opposite...gender? Sex?
Happy to answer 
It was a very long process for us. As I say trans wasn't really a 'thing' at that point, obviously there were trans people in the world, but nobody in our lives, there was absolutely no influence at all from Internet or media etc.
The way my dc describes it is as if someone has a massive nose. They are always aware that its there and it feels wrong, they try various ways to hide that part of themselves that they dislike, they can mask it for a while, but it always feels wrong and its always there. Some people may be able to live with the massive nose, but others will only be truly happy and feel like themselves if they have surgery to make their nose look different.
My dc was never really 'boyish' or 'girly' and still isn't. They have always liked various things, had friends of both sexes, worn a range of clothes etc and nothing has changed in that regard.
It wasn't really a full on decision, my dc was getting more and more unhappy and couldn't articulate why at that point, just that they felt 'wrong' despite looking like a stereotypical member of the sex they wish to be although that was more of an unconscious decision than I wish to look like X sex.
One day they were somewhere new, a kid asked my kids name and they said a completely random name, I was shocked at the time, but let it play out, and thats been my dcs name ever since.
They are only now, many, many years later beginning to undergo any sort of physical change, and they have genuinely never been happier in themselves after a lot of challenges, heartache and trauma along the way.
It was never a case of "I'm trans" more like eliminating potential causes of their feelings and somehow along the way this was the right fit for how they felt. This began for us over a decade ago now so its been some time.