Writing about something that you know is a lot simpler than writing about something you are not aware of. Same holds true for children's writing as well. To publish children's book, you must go writing about a topic you know. In case of non-fiction, it becomes extremely important to know your topic upside down. However, in the case of fiction as well, you need to have the complete knowledge of the background you would set your story in. in the absence of the complete knowledge the material would start to look lose and lacking in essentials.