I've been a freelance artist with licensed artwork since 1998, working primarily with DC Comics' Licensing, drawing all things BATMAN, SUPERMAN, JUSTICE LEAGUE, and SUPER FRIENDS. I've even done some movie tie-in kid's books like THE SPIDER-MAN 3 SOUND BOOK for Meredith Books/Marvel Entertainment, etc. I first got work by working up a few sample pieces and taking them into show DC's Licensing editors. I was giving some trial artwork to do, and based on that I began working on BATMAN THE ANIMATED SERIES coloring/activity books and later full color storybooks, etc. Here's a few things you should know:
-Children's books come in all different sizes, so there is no specific size paper that is standard, and there isn't really a standard format, as books vary from project to project and company to company.
-You should NEVER send any sort of ORIGINAL artwork to a company via a submission of any kind. Always send a copies of the work--a printout or a photocopy of the artwork is ideal. You can use glue on the original artwork(s) and text, but the copies shouldn't have any paste-ups or mechanical treatment. Just nice, clean copy images of the artwork.
-If you're going to submit copies of your artwork to companies as a submission, don't send a whole book's worth of art. Art directors and editors don't want to look through a 25 page book, and frankly don't have the time to do that sort of thing, so just a few really good pages will do fine. 3-6 pages is plenty to send, and all that is necessary. Send examples of your best and most current artwork.
-Before you submit anything though, consider this: The best thing you can do is to find out WHAT a company publishes, and then submit something similar to what they print to them. Some children's book companies do look for an original kids book with original characters and story, but many companies publish licensed books--this means they publish children's books that feature pre-existing, licensed characters like those of Walt Disney or Marvel Comics, or television cartoon characters, like Sponge Bob. I wouldn't bother sending an original idea for a children's book to a kid's book company that doesn't publish that sort of thing, because you'll most likely be wasting your time and theres by doing so. It's a good idea to learn what sorts of things each company will and won't publish before you submit samples, so go out to the bookstores and educate yourself. Check out what companies publish and submit accordingly. If you intend to submit your own original ideas, it's very possible that book companies who publish such works might actually require you to submit your work via an agent, so look into such things before you send stuff out. Most licensing publishing companies do not require you to have an agent, and you can submit to them directly. You should be able to find out the book companies mailing address in the books they publish or online, etc. Do some research.
-Make sure you include a letter of introduction with your submission, and make sure your name, address, phone #, and email information is presented with each sample page you submit, and send your samples bound in a professional looking binding. One of those clear ring bindings treatments from Staples or Kinkos will work fine.
-The key is to educate yourself about the companies before you send in anything. Once you know who's who, and what's what, then prepare your best artwork, make great copies of your art, then submit with a professional presentation. If you don't get work or land a kids book deal out of the gate, don't get frustrated. Keep submitting. If you're given advice or critiques, listen to what's said and keep that in mind for the next submission.
Best of luck,