The children's book market serves young readers of all ages, and it offers different types of publications. There are picture books, early readers, chapter books, middle-grade novels and young adult books. There are fictional books, books of poetry and nonfiction. It helps to know what age group you're targeting and what genre you're writing in right from the start. Find thoughtful readers early on -- your family and friends -- and listen to their responses, revising your manuscript as necessary.
The Society of Children's Book Writers & Illustrators (SCBWI) is a good resource for children's book authors. Local chapters sponsor events, and national conferences bring in big-name speakers to educate and encourage children's book authors. SCBWI publishes a publication guide with lists of agents, editors and publishing houses, as well as guidance on matters such as contracts. You can organize critique groups with other SCBWI members in your area, or start groups with children's book authors you've met online or through word of mouth.
Finding a literary agent to represent you can help propel your career forward, since many publishers won't even consider unagented submissions. A good literary agent will provide advice on your manuscript, be up to date on the current state of publishing and have ideas about which editors and publishers are most likely to respond to your work. Agents should not charge fees upfront to read your work; they earn their money by taking a portion of what they sell for you. Finding an agent begins with sending out query letters. Websites such as Agent Query are clearinghouses of information on agents and the process of finding one.
Whether you are working with or without an agent, you will probably consider publishers of many different sizes; everything from huge corporate publishing houses to smaller independent presses. All can serve your needs, depending on the books. While publishing houses such as Scholastic and Random House have name recognition, smaller publishers may serve your needs better. You may also consider self-publishing, an increasingly popular alternative that many authors choose instead.