Visit bookstores and libraries and see what topics are selling the best. Decide if the book is to be composed of text only or if it's to be a picture book.
Read the work of contemporary child writers. Here are some suggestions: "Fogbound" by Steven Shepard (1993), "I Was Born to Be a Sister" by Akaela S. Michels-Gualtieri (2001), "Our New Puppy" by Isabelle Harper (1996), "The Timekeeper" by Anna Riphahn (1996), "Just Peace: A Message of Hope" by Mattie Stepanek with Jimmie Carter (2006) and "Taking Flight: My Story" by Vicki Van Meter (1995). Ask your reference librarian for more titles.
Compose a list of book topics about which you would feel comfortable writing. The best authors -- no matter their age -- write about what they know. That doesn't mean you can't embellish your experiences. It's OK to write about a space ship, for instance, even if you haven't been on one. But you will need to figure out how you would feel about riding in a space ship to make your book believable and compelling.
Write your book. Some authors write by hand, using a legal pad or other writing paper, while others use a computer. Occasionally, an author will dictate a book using a voice recorder.
Create your drawings on separate pieces of paper, if you are writing a book that will be illustrated. Scan each piece of original art into a computer, then merge the art and story using page-layout or word-processing software. Print a hard copy and save a complete copy of the manuscript on your desktop.
Buy, borrow or check out from the library the most recent copy of "Writer's Market." This reference guide is the unpublished author's best friend when seeking a publisher or an agent. Peruse "Writer's Market" to find agents and publishers specializing in children's books.
Write a short, attention-getting letter to each agent or publisher. Tell them about the book. Don't send the manuscript. Today's publishing world is very crowded, and editors, publishers and agents don't have time to read manuscripts. Your letter should tell the recipients what the book is about and why they should choose your story for publication. In some cases, these professionals prefer email to a letter. "Writer's Market" can tell you what each agent and publisher wants, so stick to their parameters.
Keep writing letters or sending emails. This is the hardest part of the process. If all efforts fail and you still want to publish your book, publish on demand (POD) technology can make your dream come true. Many POD offerings are available online and include a cover design as part of the package.