SCBWI -- The Society for Children's Book Writers and Illustrators:-

The Society for Children's Book Writers and Illustrators (SCBWI)
SCBWI -- Tri-Regions of Southern California
Kite Tales -- Newsletter of the SCBWI Tri-Regions of California
Current Issue of Kite Tales
SCBWI-LA - Listserv
SCBWI-OC-IE-LB - Listserv

Useful Publications:-

Children's Writer Newsletter
Publisher's Marketplace
Writer's Digest
The CBI Clubhouse
Children's Literature Listserv - Theory and Criticism of Children's Literature
Horn Book Notes
The Children's Book Review

Agent/Editor Search:-

Preditors and Editors
AAR - Association of Author's Representatives
AgentQuery.com
Literary Rambles -- Details on literary agents for children's writer and illustrators

Useful sites to help you in your writing:-

Random Plot Points -- Gives you sticky situations to put your character into.
Babynames.com/character-names.php -- Give your characters memorable/appropriate names.

Plot

Freytag's Pyramid -- article suggested by my friends in Idaho
Plot Structure from Learn Hub
Structure and Plot
Basic Hero's Journey Plot Structure

Dialog

Diaogue, Some Basics by Michael Stearns
If your Dialogue fails, so will your story

Voice

Giving Voice by Emma Dryden

Character

Creating Believable Characters in Children's Books by Robyn Opie

Interesting Online Articles:-

Read This b4U Publish -- A thirteen-year-old boy tells the industry what teens want

Five Ways Writers and Book Publishers Need to Embrace Change NOW

One Thousand True Fans -- Developing loyal followers for your creative works

The Need for a Digital Conceirge