Oy, I'm on the fence about this a bit. But when I consider what I was doing at around 8...the thought of being that torn up about my gender identity never entered my head. For a kid to be this serious and sure about it means there's a definite basis for treatment IMO. I understand the pain that people with GID have to go through, the torture of being born in the wrong body. I can see the argument for catching it early so she didn't have to go through that, certainly.
If you have any serious concerns, are being harassed, or just need someone to talk to, feel free to contact me via PM