“I have an idea for a children’s book. Who should I pitch it to?”
I see this question often on writing forums and it takes willpower to refrain from posting, “Whoa, back it up, Nelly.”
Writing for children is hard, hard work. I do not advise attempting it without a thorough understanding of children’s literature. The best place to start is the Society of Children’s Writers and Illustrators.
Children’s literature has multiple categories based on age. There are board books, early picture books, standard picture books, easy readers, transition books, chapter books, etc. Picture books in particular have specific page and layout restrictions. There are language expectations based on target audience. You can find a list of categories at Write For Kids.
Go into a bookstore and head to the children’s section. The first thing you will notice is it is full of “classic” children’s books, the books parents of baby boomers read to them. Then there are successful series by writers such as Mercer Mayer’s Little Critter series and Stan and Jan Berenstain’s Berenstain Bear series.