There are many children's book publishers all over the world. Sometimes, authors are their own publishers as well. They are the ones who are in charge of all the publishing stages: acceptance, negotiation, edition and design, prepress, printing and eventually the privishing stage. Children's book publishers do not work alone - they work together with illustrators, photographers, editors, designers and booksellers. Most books for small children are illustrated or have some pictures.
Some writers have their texts ready to be published and be part of the book industry. To find children's book publishers, they can search on the web - there are many websites with very long lists of children's book publishers.