The most important thing any parent can do in this situation is to accept your child for who they are. It's not our job to dictate to our kids who and what they should be; rather, we should provide them with a safe, loving, nurturing environment in which they can discover who they are.
My eldest daughter has always been a typical girly-girl: during her dressing-up phase she always dressed as a Disney princess and her favourite toys were dolls. But her younger sister was gender-nonconforming when she was little: she abhorred girls' clothes and her dressing-up phase consisted of her dressing as The Hulk, Spider-Man and Ben Ten - and her favourite toy was a fire engine.
So I supported both of them in expressing themselves as they saw fit. We'd go shopping with my eldest dressed as Rapunzel, and my youngest in a Hulk muscle suit complete with a voice-changing face mask. We'd get loads of smiles from other parents as they approached us - there are few things as funny and adorable as a tiny two-year-old in a huge green muscle suit.

But then they'd walk past us & look at my youngest from behind: they could see that she had long hair (and was therefore either a girl, or a boy with long hair). Their faces would immediately turn to thunder and they'd shoot accusatory looks in my direction. How dare we let a girl dress as The Hulk? Or how dare we let our boy grow his hair long? What kind of parents were we?!?!?
The best kind.Because that's my child, and that's how she wants to present herself to the world. She's thrilled with her Hulk costume and she knows she looks brilliant in it. So that's what she's going to wear. Anyone who doesn't like it can quite frankly do one.
But also because I was struggling with my own gender issues. I remember what it was like to be forced to wear girls' clothes when doing so made my skin crawl. I remember looking longingly at my brother wearing a Superman T-Shirt when I was forced to wear some flowery rubbish. I had no way of knowing whether my daughter was a tomboy or trans - all I knew was that she preferred boys' clothes and wanted to be the Hulk. So that's what we did for her, knowing that the truth of who she is would be revealed in the fullness of time.
She outgrew her phase and settled into a female identity several years ago. She's now in her teens and whilst still not a girly-girl, she's definitely a young woman. No harm has come to her from letting her express who she is. Rather, it's helped form her into a strong young woman who knows what she wants.
But if she didn't outgrow it? If it actually turned out that my child was trans? We'd have been down the Tavistock getting puberty blockers faster than a speeding bullet. I wouldn't wish what I went through on anyone.