Writing books for adults and writing books for children are two very different things. While it may seem rather asinine to say that, there are some folks out there with the mistaken apprehension that writing for children must be easier than writing for adults. "It's shorter! You can write about silly and ridiculous things! And a picture book? *phffftttt* Child's play!"
In actuality, most people agree that writing for children is harder than writing for adults. Children's developmental needs require that one write with humor, kindness, respect, wonder, and keep it short and to the point. Don't talk down to children, or fail to take them seriously. And if you're writing a picture book, don't forget that you are in fact working in a visual medium.
There is a lot at stake.