Some magazines pay on publication. That means you won't be paid until your story appears in print. This could take a year or more. A good idea. Barbara Seuling teaches a class on AOL in Children's Writing.
Many colleges offer classes as well.
Write to Sue Avon-Wack for details about Group Bench W, an online critique group. I'm thinking about taking a correspondence course from the Institute for Children's Literature. The organization is legitimate. Opinions on the value of the lessons varies. The Society of Children's Books Writers and Illustrators is a national organization for writers and artists.