I also started seeking publication of my writing at age 12 and simply sent stories around to magazines to get my foot in the door. I wrote children's stories, but I sent them as an adult writer and was published (and paid--what a thrill) at age 13. I followed the advice of books such as The Writer's Market
and some books on writing for children and teens by Jane Fitz-Randolph (they are now probably somewhat anitquated). TWM
should give some advice on finding an agent. I did not seek out an agent, but I was not trying to sell a book. I imagine that if your DD does not tell potential agents or publishers that she is a child, they will be less likely to dismiss her immediately, but things could be different these days.
If, on the other hand, you want to draw attention to her age at the outset, there is a contest every year for children who write and illustrate a book (called something like the young writers and illustrators contest), and the winning book is published. I don't have a link to the contest website, but here is one of the books:http://www.amazon.com/Who-Owns-Sun-S...0942234&sr=1-1
On an interesting note, Ally Sheedy published a book with a friend (who did the illustrations) at about age 13--I think her mother was a literary agent in NYC, or had some such connection--and she later went on to become an actress. The book is about Queen Elizabeth I and is called She Was Nice to Mice
. Also, I believe the incredibly successful homeschooled author Christopher Paolini, published as a teen, first printed and sold his book through his parent's own home publishing company. After it was a success, a major book publisher picked it up. Here is a link to an early HEM article about Paolini's experience:http://www.homeedmag.com/HEM/192/mapaolini.html
Hope that helps!