#3 Less is more
In books for the very young say as much as possible with a few words. Illustrations have a high literary value. They really speak to the reader.
If you develop your stories with text, write freely first, then each time you revisit your story craft the maximum meaning into fewer and fewer words. If you prefer to work visually, sketch as much as possible until the images start to pick up some of the slack. The reason they are called picture books is because the words and pictures are equally important. More than that they complement each other and together create a new language.
Let’s break that down; Some people write on their own, some illustrate, some do both. Publishers can start the process from either point but tend to work with words first more often. They will read the text and then make a decision about the illustrator based on their company’s style.
Writers, your part of the book project is going to have other hands and brains joining in. It might be a good idea to write with that in mind.

Normally, illustrators are handed a text and asked to interpret it. Some of the decision-making is already done. The illustrator’s main challenge is to interpret the text in a fresh new way, to listen to the comments and guidance from the book designer or art director. Try using the rule that carpenters use: measure twice, cut once. By the time you get to producing the final piece you will have sketched all the possiblilities you can imagine.
For creators who do both, the challenge is all about discipline. It’s really hard to wite and draw at the same time! Plus you have to be honest enough to look at what youve done and say if its good enough… or not. So do one first, then the other. Repeat until you have what you want.