Marlo Garnsworthy, an illustrator, editor, and teacher who has been working with writers for over fifteen years, has a very good post at her blog, Wordy Birdie, called So You Want to Write for Kids: The Least You Need to Know. Read every word.
The parts I found particularly interesting:
Know Your Audience--Know the Genre. It's not unusual to hear of writers with a new children's book who know little about children's literature, itself. I've read recently about people wanting to write children's books because they enjoyed it as a child, but they don't read it now so they just don't know what's happening in the field. I really love hearing about authors of YA books who say in interviews that they just wrote what they wrote and their editors/publishers decided it was YA. You really can't depend on something like that happening. On top of that, my gut feeling is that people should know what they're doing, no matter what they do for a living. That includes knowing that you're writing YA.
Think Story, Not Message. How many adult readers tolerate reading message books? Why do so many people think it's a necessity for children's books? Teachers teach. Preachers preach. Writers should stick to stories.
Learn. Marlo says, "One of the things that always surprises me is that newer writers think they should automatically know how to write a publishable story." I'm extremely embarrassed to admit that that was probably the case with me when I was getting started. But I was wrong. Even before I finally got my first book published, I realized I needed to learn more. I needed to learn more after the first book was published and after the second book was published, too. My seventh and eighth books have some structural problems I regret. I should have known more. I have spent the last ten years studying and changing how I write.
I am quite taken aback when I am in elementary schools and teachers ask where their students can submit work for publication. A few years ago I was in a school and students were telling me their parents thought they should publish. To tell children they they know how to write a publishable story, to let them believe that, is such a disservice. Show them how they can learn how to write.
Get to Know People. This is more necessary now than it was when I was getting started, in large part because so many more people are trying to get into writing. Especially in children's writing there is all kinds of networking going on, and gatekeepers will remember names of people they've met or heard about through others they know. It won't get a bad manuscript published, but all things being equal, it could get that last bit of attention that makes a difference in who gets published or who moves up the ladder because s/he is known. On top of that, nowadays if you have a network of literary friends/contacts, those people will help promote your newly published book in many ways.
Marlo covers a lot of material in her post, and there's a reason for that. You really need to know a great deal in order to have a chance of getting into the publishing world.