If you are beginning a writing career, I would not recommend starting with books. I started my writing career 30+ years ago by writing church school take-home papers. This is a wide open market. Most publishing houses use about 60% freelance material in their take-home papers. Many denominational as well as nondenominational houses publish take-home papers for every age. These come out weekly—fifty-two times a year. There are many opportunities here to get published and to continue to sell your material as reprint rights over and over once it has been published the first time.
There are many things you can sell to the take-home paper market. Children’s fiction, particularly for 4th-6th and junior high is very popular. These are normally about 1,000 words in length. How-to articles, nature and domestic animal stories, and paraphrased Bible stories are included here. These are shorter, about 500-800 words. Check your Writer’s Market for the correct length for the particular publication you have in mind. Also, crafts, puzzles, and activities find a market here. Many adult periodicals print children’s stories as well as children’s magazines. Most children’s materials will not require a query letter because the manuscripts are so short, but check your Writer’s Market. Some magazines will require a query, but rarely will a take-home paper. Also consider writing for the online publications for children.
Once you become successful at selling to these markets, you might consider moving into curriculum. Curriculum is usually done on assignment by publishers, but they often use freelancers to do the writing. Check your denomination and see if they publish curriculum. Normally you would be assigned a quarter’s worth of material. It would be helpful to teach at the age level you are writing for and try out your material before submitting it for publication.