What it's saying is that because she likes children alot, she (must have/was probably never/could never have been/wasn't) a teacher. Despite the strength of the language its just speculation by the speaker. He or she said this because in their experience, if this other person HAD been a teacher at some point in time, she would naturally dislike children probably for reasons such as children are noisy, troublesome, disobedient, etc. Her love of children is not tainted by the negative experience of having been a teacher.