Adults buy children books. Children books must appeal to adults who buy them for children. Usually children influence the decision, and well executed illustrations in children books get chosen more often than otherwise.
The children book illustrator can add a bit of humor to their drawings. The illustrator can be considered like a book designer. The illustrator must come out with pictures based on the text of the book. The pictures must compliment the words to form a coherent flow. A simple story may not have very detailed description of the scenery, costumes, people and other things. A good illustrator helps the children to visualize.
Color is not always demanded although it is good to know how to color using different types of medium. Children books are not confined to fairy tales. Other books like riddle books, joke books, puzzles, and activity books use the services of the humorous illustrator.
When trying to sell to publishers, concept is important. The drawing is secondary. Make a sample book out of 16 pieces of typewriting paper. Choose a story and work on it keeping the target audience in mind. Sometimes you might even have to write the text.