If you want to be a published writer of children's picture books, your work is not done when you write the final word of the story. Typically there are submission guidelines for a sending a children's book to a publisher, and you're expected to adhere to them. Picture book authors also have to leave room for an illustrator to tell his story alongside theirs and must write at a level appropriate to the targeted reader. If you carefully follow all of the publisher's instructions for submission, you have passed the first test.
Consult a market guide to select a publishing house and an editor to receive the manuscript. You may wish to join the Society of Children's Book Writers and Illustrators, whose members have access to regularly updated industry information including a list of which titles have been published recently by houses, which will help you get an idea of what types of books they publish. Make sure to send it to the appropriate place -- do not send fantasy to a nonfiction publisher or a children's book to an adult publisher -- and whenever possible, specify an editor by name.
Using the market guide or the website of your chosen publishing house for submission requirements including word count, margin specifications and inclusions, prepare your manuscript.
Print the manuscript according to publisher specifications such as word counts and to industry standards for margins, fonts and spacing.
Type a one-page cover letter to the editor. In the first paragraph, describe your manuscript, including targeted reader age. In the second paragraph, explain why you chose this publisher. In the final paragraph, state your writing experience. If the manuscript is nonfiction, specify why you are qualifed to write this book. Be brief. Some children's books are not even as long as the letter, and you want the editor to lift the cover letter and be wowed by your first line.
Print, proofread and sign the letter. Paperclip the letter on top of the manuscript. If the publisher accepts email, you still need a cover letter but save the postage.
Address the envelope and affix proper postage. Put letter and manuscript into envelop, seal and post. If it is an email submission, address your email to the proper person, attach your materials and send it.