The most important detail to keep in mind when preparing to write anything is your audience. Writing must be tailored to the audience it is geared toward targeting.
That being said, a children's book must a) be written with terms a child can comprehend and retain, b) include illustrations for the purpose of keeping their attention and encouraging retention of the information
As with any book, the first key component is the writer's passion. If this is missing, the book will not be successful. One must possess a great passion for writing and an understanding that it is important to see the book from the audience's eyes! (i.e. look at the book from a child's perspective).
One does not need to possess a PhD to write a book - just passion and dedication.