Decide who your reader will be including their age and sex. Narrowing your target audience helps give focus to your story and lets you think of that audience as you write.
Consider if your book will have a message behind the story or if it will be for entertainment purposes only. Most successful children's books manage to be both educational and entertaining.
Choose a voice for your story. Once decided, stick to a voice to increase cohesiveness in your writing.
Determine whether the story will be a character- or plot-driven story. This means deciding between following the events that happen to the main character and watching the character react to the events taking place in the plot.
Select the time of day that the story takes place and how that will determine the language used, the events that take place, the weather and setting of the story.
Know whether the child will read the book or have the book read to them. This will help you determine the length, language and number of words on each page.
Start drafting your story with all the above considerations determined and don't forget to have fun. If you enjoy writing it, children will enjoy reading it.