One of the most difficult things about breaking into children’s writing is finding the right market for your manuscript. Many new writers are in such a hurry to be published they send their manuscript out to as many publishers as they can and often before it has even been edited properly. This is a big mistake.
It has also been recommended that you don’t write a word until you have a firm contract. This might be true of educational work but, less true for fiction. Even so, it is worth writing and asking for any writer’s briefs. That’s information about projects they are developing and not their underwear.
It's important to research your markets regularly. Don't send picture book manuscripts to publishers that only print educational non-fiction, or primary school educational non-fiction to a publisher that focuses on secondary education. I know this seems obvious, but we hear over and over how writers send their work to inappropriate markets.
Research the different publishers and what they produce. Libraries will give an indication of books they have previously published. But, remember to request their new catalogs, or go to the bookstore and browse. Make lots of notes. Look at the titles and the title page with current editorial contact information on. Pay close attention to the focus of the books. The more time you spend in this preliminary research, the more likely you'll be to find the right publisher for your work, and you'll spend a lot less on stamps.