There are a few things consistent about writing and submitting to a children's book publisher in addition to specific guidelines different publishers may have. These rules should be followed if you want to have the best chance for success. Always read specific guidelines and be sure to follow them. Format your manuscript correctly, be sure to use the correct publisher name and always include a short letter of introduction. Do not tell the publisher how great your book is. Simply let them know about any previous credits you may have (briefly, and only those credits applicable), briefly give an overview of your book, ask that they consider it for publication and thank them for their time. These are basic guidelines that should be followed in addition to publisher-specific guidelines.
The "Children Writer's and Illustrator's Market" is one of the oldest and most-respected resource available for finding markets for your book. It is updated annually and gives detailed submission guidelines for hundreds of markets. The guidelines specify pay rate, where and how to submit and other important information. Go through the book and choose a few markets that seem like a good fit for your work. The book is available on-line and in most bookstores (see Resource 1).
There are several on-line market databases that provide detailed and updated market information for authors of children's books (see Resources 2 and 3). The Colossal Directory of Children Book Publishers is one of the best. Also, Writer's Market Books offers the Writer's Market database on-line for a small fee, which includes markets for children's publishers. If you're serious about making a living in the business of writing children's books, the Writer's Market is a good investment.