This book is updated each year and can be found or ordered at most bookstores. Also look for books that specialize in the children's market. I recommend Twelve Keys to Writing Books That Sell by Kathleen Krull. Check out the following web sites for information regarding children's book publishing. Take a writing class at a college extension program or community college.
Some classes deal specifically with writing or illustrating for the children's market.
Consider a professional, reputable critique service for finished manuscripts.