Get a copy of
Children's Writers and Illustrators Market. Like the regular
Writer's Market, it's published every year, but as it deals strictly with book and magazine markets for children's and YA authors, it's a much more manageable size.
Also, the book's author / editor, Alice Pope, keeps a blog where she posts any updates or additions to the markets in the book. It's a great resource:
http://cwim.blogspot.com
Also, if you join SCBWI (Society of Children's Book Writers and Illustrators), they'll send you a fantastic markets guide. It lists the acquiring editors at each children's and YA publisher / division / imprint and has their submissions guidelines. There are still a few publishers that accept unsolicited submissions -- usually they'll specify that they want to see a query letter and a few of the novel's opening pages first, just like many agents want to see.
Good luck!