It is wonderful that your daughter enjoys writing so much. You have received some good ideas! I was a prolific reader/writer when I was young and had some nice encouragement along the way. Here are some of the things that helped me that come to mind.
My aunt was a big cheerleader, frequently sending me information about things like how to copyright one's work, quirky poetry (her own and writers like Shel Silverstein). She made me feel like I was a writer and would grow up to be a great one if I kept at it. (I didn't, lol. Or at least haven't lately.)
My mom is an excellent editor and was very patient in editing my work, but more importantly, explaining to me how to make improvements on my own. Books like "How to Edit Your Own Writing" have been helpful to me. Learning to outline, while slightly annoying, taught me how to structure my thought and arrange/rearrange before I'd gone to the trouble of writing the story/article.
I just saw some reviews that were very good for a book called "Guiding Readers and Writers." She likely won't need the readers guidance, but you might consider checking it out for the writing portion (it is aimed at educators).
