The first thing you do in writing a children's book, is to understand and think like a child. When we were kids, how did we think? Remember that a child's mind is free from the rules, restictions, and processes we learn as we grow. The implausible is plausible. With a child's imagination, anything is possible, so things like logic can be kept at bay.
The second thing when writing a children's book is coming up with a title that appeals to a child. Something that is catchy, but intriguing and speaks to the age and possible gender of the child. Take a theme, object, or character from your story and go from there.
The last step in writing a children's book is to remember that children don't want to be scared. Don't write something that going to give them nightmares. Write something that going to be positive and give them positive images and reinforcement.