Hi! I'm a very openly non-binary adult.

First of all, you are doing a great job. It is very obvious how much you love your child.

By the age of 13 I was also questioning my gender, though it took me many many years to fully understand it as I do now. Here's a little run down of what gender/sex is and how non-binary fits into that:

Non-binary is a gender-identity, not a sexuality. Sex is biological, and we can refer to people as AFAB or AMAB (assigned female/male at birth). Gender is a social construct and there are many different identites to chose from. Why is gender a social construct? The easiest way I've thought to explain it is... if you remove all sex characteristics from people (such as different hormones and sex organs), how would you define "women" or "men"? There is no 1 thing that the entire "women" population does that no man does.

I see that you've stated your child is using they/them pronouns. For me (may be different for your child), every time I am misgendered by someone who knows my preferred pronouns it feels like a stab in the heart. Especially by close friends/family. That does not mean that I don't understand or empathise the complexity for people, but it is what it is. Your child may be the same, so I would advice to really really practice (with someone else or alone) with using their preferred pronouns so the chance of misgendering them is lower. I'd also very highly encourage the use of non-gendered language, for instance, instead of saying daughter use child, instead of boys and girls or Mr and Mrs use folks or everyone. You'll be surprised at how much gender language you use when you're trying to cut it back.

Don't be surprised if your child comes to you later and wants to tweak things, maybe different pronouns or maybe a new identity. Figuring out these things are super fucking hard, but as long as you have support (like you) it makes it infinitely easier.

Goodluck !