I have a sim that is a writer and one that is both a writer and artist, they are both making lots of money.
My suggestion is to have her read as many of the books as she can, it helps in her writing. Read a book in each genre and then go back and read another one in each genre. That way, by the time she is an adult, she will probably write better novels. I didn't do any formal testing, but it did seem that an earlier sim author that I had deleted hadn't read any of the books and it took her longer to get the good pay and have best sellers.