As with any type of writing, successful children's book authors always write to a specific audience. The term "children" is broad and as a children's book author, you must narrow it down considerably. Determine what age group you are writing for and understand the typical comprehension and vocabulary levels of that group. Consider these elements when you are composing your story and share it with children in your target age range as a test for its suitability.
An important factor in writing a children's book is coming up with an idea that is unique and stands out from what is already on the market. Go to libraries and bookstores and read some children's books written for your target audience. Speak to the person in charge of the children's book section at your local bookstore and library to find out what is popular among children in specific age groups.
Feedback is an important part of the writing process. Joining a writing group made up of others who are also writing children's books provides valuable feedback and learning opportunities. Attending conferences and joining professional organizations like the Society of Children's Book Writers and Illustrators helps you keep up with the current trends in children's writing, learn from other children's book authors and give you important information on the publishing process.
Always write to the best of your ability, regardless of what age you are writing for. Just because you may be writing for children too young to read does not mean your writing should be lacking. Strive to always write consistently and ensure that your story's entertainment value is not based solely on rhyme or the accompanying illustrations. Be conscious that your writing "shows" rather than just "tells" and that the concepts about which you are writing are age appropriate.