One of the many responsibilities of a children's book illustrator, is to keep the characters in your illustrations, consistent. This is also one of the most, if not the most, difficult to do! The shape of the head, body,... actually everything must stay the same throughout the whole book. Eye color? Hair color? Pudgy cheeks? YES! Even the style of the artwork, as well. Would it not be confusing to have the beginning of a book painted in a carefree, fun, watercolor wash and then have it end with realistic, tight, oil paintings?
I find I ask myself several questions along the character development stage. How old do I want this character to be? How tall? Skinny or not? Freckles? Once Ive decided on the basic character traits, many sketches are done for each pose and each page of the book. I find it very important to review the character throughout the book. Unless the character had a hair cut, hair dyed, lost or gained weight, got sunburned, etc., the character on the first page of the book, should look like the same person/character on the last page of the book.
Here's a few paintings I've done recently. I won't go into details about the story behind the illustrations because the book's not published yet, but, hopefully, you can see that this is the same child (and cat) in each illustration.
And here's how the final page will look.