Sometimes a person who wants to write for kids approaches me as a librarian and asks for advice. And, as a librarian, the best advice I have to offer is to READ!
Want to write picture books? Read picture books. Want to write historical fiction? Read historical fiction. Read anything and everything in the way of children's literature. Read books, magazines, websites designed for children, poetry, activity books, etc. Have I made it clear? To be a writer you have to be a reader.
If you're writing for your eyes only, then of course you don't have to read, but if you're coming to someone for advice, then you want to reach out to readers, so...you too, must be a reader. What else besides children's literature should you read? I'm glad you asked.
Here's a very short list of items that I've found to be of use:
Bernays, Anne, and Pamela Painter. What If? Writing Exercises for Fiction Writers (HarperCollins, 1991).
Brohaugh, William. Write Tight: Say Exactly What You Mean with Precision and Power (Sourcebooks, 2007).
Underdown, Harold D. The Complete Idiot's Guide to Publishing Children's Books (Penguin, 2008).
Join the Society of Children's Writers and Illustrators, SCBWI, and read all the information that they provide.
Buy a copy of the latest year's Children's Writer's and Illustrator's Market (F & W Media). Study the market information, but also read the accompanying articles that contain practical information and writing tips.
That should be enough to get you started! (And I haven't mentioned the information that's available online!)
If anyone wants to suggest a favorite writing book, please comment below.