That was my sibling, though in their early twenties.
Very gender critical, generally 'critically think for yourself' upbringing that we both shared.
My sibling was never one to submit to peer pressure. Flouted norms and expectations left right and centre. Wore hair and clothes exactly as they wanted, not as anyone else would expect.
Past the teenage rebellion, she just wanted to live, without always making a statement. Unfortunately, living the way she felt comfortable with necessitated 'making a statement' as it didn't fit into any expectations.
From at first 'coming out' as 'trans', she then spent about 15 years trying to fit in somehow without actually 'transitioning' - because she was so gender-critical. She felt, rationally, she ought to be able to live the way she wants (that sounds a bit entitled; I mean simple things, like, living free of harrassment and without constantly having to explain yourself) without having to alter her body. She fought for this (on a grassroot level). She felt that society's expectations towards gender are in need of change, not her body.
But unfortunately she found that living a quiet life when your behaviour and appearance don't match what is socially expected of your legal & biological sex isn't really possible in our society, unless you are prepared to continuously be making a statement, and submitting yourself to query, criticism, and ridicule.
The attempt was so stressful, it made her mental health deteriorate massively.
In the end, she couldn't take it anymore. Surgery was the only way to get her papers (bus pass, driving licence, bank papers, passport, ...) in line with the life she was living, the only way that allowed her to 'just live'.
For this reason, I support some kind of (well reflected and thought out) form of self-declaration. My sibling would not have had to have surgery if that would have been an option.
For this reason I furthermore do not think that 'trans' is a medical issue per se (though I suppose it can be). My sister was not 'ill'. It was society's gender expectations that caused her issues. There is nothing 'ill' about a male bodied person desiring to live, peacefully, in ways that are associated with female bodied people.
On this background, if either of my children consider themselves at any point to be 'trans' of any kind, I would totally support them in fighting for changes in society, towards the end that they can live the way they feel comfortable, without having to change their bodies. In fact, I continue to fight for this myself, and will support my children in fighting against gendered expectations wether they have trans issues or not.
But if either of them should have 'trans issues' I would equally try to show them that they sadly cannot expect huge changes in social expectations, and though it is a worthy fight, it is not their responsibility alone to change society. So if it becomes more than they can take, I would totally support them in finding a way to 'just live'. If that means changing their bodies, I would feel very angry that we live in a time and place where that is the only way, but I would support them every step of the way nonetheless.