How does one know what is appropriate for an age group or grade level? Read! If you want to write a chapter book, read 100 chapter books. Find a fourth grader, or a teacher, and ask to look at a fourth grade textbook. Check out the choice of words, the length of the sentences, etc. Invest in a copy of the Children's Writer's Word Book by Alijandra Mogilner (F & W Publications,2006).
Another book worth looking at is The Reading Teacher's Book of Lists, fifth edition (John Wiley & Sons, 2006) by Edward B. Fry and Jacqueline E. Kress. I know its title implies it's for teachers only, but the material in it is of great value to a children's writer. I have an older edition that I like to consult for its list of "Instant Words"--
These are the most common words in English, ranked in frequency order. The first 25 make up about a third of all printed material. The first 100 make up about half of all written material, and the first 300 make up about 65 percent of all written material. Is it any wonder that all students must learn to recognize these words instantly and to spell them correctly also?The "Instant Words" list in invaluable when attempting to write a beginning reader or early chapter book.
The "Descriptive Words" list is good to browse through instead of always reaching for a thesaurus. I love the list of "Collective Nouns." It contains not only the terms for groups of animals, but terms for groups of people and things. For example, a group of performers is a troupe; bacteria form a culture.
My edition of The Reading Teacher's Book of Lists, contains 194 different lists, and the fifth edition has more than 200!
If you use Microsoft Word, the program has a readability feature to help you find out if you're on the right track when you're writing for a specific grade level. If you use another word processing program, which doesn't have the feature, you can check your text online. Do a Google search on "readability check" for more information and links.
By all means write your story first before trying to make it fit into a grade level. The story should always come first. If you don't tell a good story, your audience is not going to read it no matter how easy it is to read!