This reads as transphobic, but I'm going to leave it up for two reasons:

• First, your subsequent comments don't show any malice, just a lack of understanding.
• Second, that lack of understanding appears to have been rectified by others' comments, and I think seeing that could be valuable for onlookers.

I'm not trans myself, but my son is, and he explains how he felt something like this...

Imagine you're a pretty average boy, and interested in stereotypical boy things. You like boy clothes, you want to hang out with your friends who are boys, you look forward to growing muscles and a mustache when you're old enough, etc.

Now imagine that at the same time, all your clothes are girl clothes, your name is girly, and everyone around you thinks you're a girl.

That wouldn't match up, would it. It would feel incongruous and wrong.

If you're cisgender, you can absolutely prove you're a boy to everyone's satisfaction, and you're unlikely to get dressed in the wrong clothes or given a mismatched name in the first place. So you probably don't ever have to think about your gender and whether it's being expressed by your body incorrectly.

But for trans people, it's not uncommon for them to become acutely aware of gender when they're young in ways that their cisgender counterparts don't, precisely because of the mismatch.

That's how it was for my son. He didn't figure out he was trans until he was around thirteen, but he knew something wasn't right long before then.

full disclosure: I'm trans - my story at the end.

It doesn't really work that way... if she was told "you're trans" and wasn't, she would experience the opposite type of gender dysphoria over time and would want to express masculine/boy identity in line with her birth gender and physical traits.

She's young enough that it's not infeasible she might change her mind. Ideally she has been to a licensed therapist and/or psychologist who verified she met the latest DSM diagnostic criteria for gender dysphoria (the only successful treatment for which involves transition), and they are helping the parent manage this process. They'll be in no hurry for medical intervention since puberty would be 4-5 years away most likely.

That being said, if you meet enough trans people, it's not uncommon to hear stories about "knowing since I was 4". Oftentimes it's accompanied by a regret that their feelings were suppressed and they had to go through the torments of puberty "in the wrong body" as it were.

My own story is different. I didn't "know" until my thirties, even if I can see signs going back to childhood in retrospect. Apparently at three years old I told my mom "I'm a butterfly and I'm going to be a girl". She never told me that, I found out after she died last year, looking through her old notes. She assumed I meant reincarnation. Ironic considering I have adopted a butterfly metaphor to express my transition to a woman.

Anyway, yes, an eight year old can absolutely tell adults they are a different gender than they were assigned at birth. Whether they literally say "I'm trans" is kinda beside the point.