Writer e-lists are a great professional boon and/or distraction from one's work.
I'm a fast reader, so it doesn't take much of my time to skim my lists and respond to those topics of particular interest. They've introduced me to many of my favorite fellow authors, and I've learned a great deal about the publishing side of children's/YA literature.
If participating doesn't distract you from your writing, I recommend joining. My only caution is the same one that applies to industry functions called "dinners" or "receptions" or "luncheons" or "conferences."
You may be out with your friends and you should be having fun, but you're still in a professional context. Children's/YA publishing is a small community. To the extent possible, sidestep flame wars. Stay professional. It can be too easy when sitting in front of a computer screen to forget there's a person (or 1,000) on the other end.
Children's Writer: an open-membership e-list. Children's Writer was the first writing e-list I ever joined. It was through this list that I met a number of my friends, including one of my early mentors Jane Kurtz. Jane later invited me to join an invitation-only list of more established writers and it was there that I first met my agent.
Children's Writing Biz: an e-list moderated by author Anastasia Suen.
Nancy Miller Illustrator: a portfolio site. I received an email from Nancy requesting a link from my site, but she's not (yet) published. However, her work is great. Especially if you're an illustrator, editor, or art director, surf over and see what I'm talking about.
Ursula Nordstrom Fiction Contest: sponsored by HarperCollins and "open to U.S. writers over the age of 21 who have not been previously published. One winner will receive a book contract for a hardcover edition, a $7,500 advance, and a $1,500 cash award." Submission dates are March 15 to April 15. The award will be announced June 15. See link for rules and more information.