There's a free online writer's conference focused on picture books, middle grade, and YA going on right now at http://writeoncon.com.
You can see videos, read about editor and agent preferences, etc. Here are just some of the topics:
Questions to Ask Yourself Before a Revision by editor Kendra Levin
Illustrating Children's Books by author/illustrator J.H. Everett and Studio 5
Plot and Pacing Part 1, Part 2, and Part 3, by author/literary agent Weronika Janczuk
Myths and Misconceptions by literary agent Holly Root, and editors Molly O'Neill and Martha Mihalick
Do's and Don't's of Querying by literary agent Kate Testerman