I am an aspiring published children's author/illustrator. Yup! It's true!! :) Not sure if you knew that, but there it is. :) Here are two books which have helped me immensely with regards to writing for children.
I've been writing all my life, but have no formal writing training (aside from creative writing classes). I do work really well within a framework, though, and I really believe in having the knowledge of fundamentals and groundwork for any creative endeavor.
These books show you the basic writing frameworks for different genres in easily digestible formats (which also happens to be fun to read). Both have been indispensable to me and I'll bet they will remain open and occupying whatever available counter space there is in my studio for the foreseeable future.
(Note: Please do not be fooled by the titles. The material is not shallow. They are user-friendly, and a great introduction to your future children's writing career!)
Writing Children's Books for Dummies
Lisa Rojany Buccieri and Peter Economy
Very easy to navigate, covers all the basics. A fun and educational read. (You are not a dummy if you read this book. You are a smartie... But you WILL learn a bit about dummies, ha ha). You will refer back to it time and time again. Excellent index.
You Can Write Children's Books
Tracey E. Dils
If there was a superhero of reference books for style in children's writing, this might take the title. I have found so many answers to nitty-gritty writing-style questions between these pages. Stuff you didn't even know you needed to know is here. And all within a slim (gasp) 120 pages. The lady knows how to edit, too!
I feel I'd be amiss if I don't mention The Complete Idiot's Guide to Publishing
Children's Books, Second Edition by Harold Underdown of Purple Crayon fame.
I don't have this book (yet), but it is written by a very well-regarded and respected veteran who really knows his stuff, and it's gotten excellent reviews. It is on my Amazon wish list.