If you write for children and haven't made it "official," or if you think you might like to write for young readers in any way (rebus, magazine, picture books, picture story books, easy readers, chapter books, middle grade or young adult fiction) then you should join the national organization: SCBWI (Society for Children's Book Writers and Illustrators).
Then, you are a card carrying children's writer. SCBWI sends you a host of materials and, perhaps more importantly, access to your regional chapter. Even if you live too far to attend any or many of your regional chapter's events (workshops, agent days, annual conferences featuring editors, authors and agents) you can still get access to their newsletters, craft columns (writer's craft, mind you!) and critique groups. You could even start your own local critique group with other local SCBWI members.
When you submit your work, thoughtfully and in proper format, to agents/editors, you will state on your cover letter that you are a SCBWI member. It is a sign of your professionalism. To live up to the expectations that establishes, you need to do your homework on submissions, read the websites, and do everything it tells you to do to ensure your submission will be read.
This plug for a great organization is brought to you by one happy camper, who spent all day yesterday a few hours from home attending a SCBWI workshop with Cheryl Klein, the incomparable editor at Arthur A. Levine, she of the last 3 Harry Potter novels fame (and A Curse Dark as Gold by Kansas' own Elizabeth C. Bunce, one of my favorite reads of last year, and of course many other books too), and author of Second Sight (about revision for writers).
Buy the book! For less than twenty dollars, you can experience a paradigm shift.
18 hours ago