Like Maurice Sendak, Neil Gaiman, and J.R.R. Tolkien, I do not believe there is such a thing as "writing for children." I see myself as writing for intelligent, sensitive readers of the fantasy genre -- of all ages. My protagonists might happen to be adolescents, only because the story I wish to tell relies upon it.
I have dubbed myself a middle-grade author sheerly for marketing purposes. Perhaps if I become as big as the authors I've mentioned, I can cast aside that label, but until then I have to put up with it. Luckily for me, I found some great advice from C.S. Lewis on how to (and how not to) "write for children."
As far as "how not to" write for children, Lewis talks about books on the market that are strategically constructed to appeal to a large cross-section of the target audience. I've flipped through some of these books when I've come across them on the shelves. For starters, there's just something cold and manufactured about them. As you read more closely, you also get the feeling the writer either doesn't respect the genre, their target audience, or both. They dumb things down for their audience and grossly underestimate the depths their audience is actually capable.
Lewis recommended writing to a particular child, such as how Tolkien wrote for his own children. I see myself as writing for the child inside me. I write stories that I would like, which happen to be fantasy stories that have a rich emotional core and deal with relatable issues while taking the characters (and the readers) on an amazing journey. One of the greatest compliments I've ever received (and on two different occasions) was from non-fantasy fans who've told me how much they enjoy my books. I was able to give them something to connect to that transcended the genre.
I must be doing something right, because despite being a "middle-grade author," I have had teens and adults tell me how much they enjoy my stories, and as C.S. Lewis also said (and I'm sure many other authors will agree) a children's book that can only be enjoyed by children is a bad children's book.