some things are wrong
Today i was looking at my Spanish stuff and redoing it, I was answering a question but the answer was wrong grammar it had said "she is not a girl" so i was confused.
The grammar is fine. Do you mean it is not a logical statement? In your example "girl" = "child" . " she ( adult woman) is not a child" is the logic.
It's sounds like that, but it seems to me that it meant 'she is not a girl' - but rather a woman:)