There isn't a writer alive who hasn't wished for some sort of literary oven in which to place their manuscript, knowing when the timer goes off it's done. Unfortunately, there is no such objective measurement for good writing. Therefore, the best advice I can give is to get as much input on your completed manuscript as possible before putting it in the mail.
Writing is a solitary endeavor, with authors carefully guarding their ideas and feeling tremendous ownership of the finished project. As they should. But in order for a manuscript to become a book, it has to pass muster with many people, from editors to sales reps to accountants to art directors. Once in book form, those ideas must then appeal to reviewers, bookstore owners, librarians and consumers, not to mention kids. No matter how stellar you think your writing is, if others don't share your opinion, your manuscript will never make it farther than your file cabinet.
The first "second reader" of what you write is you. You need to remove your author's hat and adopt the reader's viewpoint. You can't do this as soon as you've written the last word of the manuscript. Put some distance between yourself and the project. Take time off, start working on something else. Then read the work and try to measure it against what you consider to be high standards. Does the pacing compare to a published work in the same genre by an author you admire? Are the characters as fully developed as those by acclaimed authors writing for this age group? Does the dialogue actually sound like words real, live people you know might say? If you wrote an outline for your book, compare the finished plot to what you intended to write from the outline. Did you leave out any important elements? Did you add anything that's unnecessary? If you're writing nonfiction, did you do enough research, or did you have to pad areas with "filler"?
Then, take your book out into the world. The first stop should be your writer's group. Ideally, this is comprised of people who are all writing and/or studying children's books. Listen to their comments and take them seriously. You don't have to make any suggested changes, but you should consider the reasons for changes offered by the readers. If more than one reader doesn't understand a plot twist, doesn't believe a character would act a certain way, can't accurately visualize a setting, it's your problem, not theirs. It doesn't matter how inspired the idea is inside your head; if you can't accurately communicate this idea on paper, no one will ever pick up your book. This process of frank editing and honest critique can take several rounds before the book is "done." Ending the process too soon will only lead to frustration and rejection letters. Declaring "I just have to find an editor who understands my book" can be just as bad. Yes, a good author/editor fit is important to the success of any book, but once you decide you want to get your work published, it ceases to be a solitary exercise. You have to know that your story makes sense to other people, and the only way to do this is to get input from outside yourself and consider it carefully.
In the end, no book is ever really finished. I've spoken with many published authors who wish they could take back their books for one more run though the editing mill. Every time we write we improve our skills a little more. One final aspect then, of knowing when a book is done, is learning to walk that fine line between making it better and letting it go. Learn to recognize when a manuscript is as strong as you can make it, and then send it out and begin your next masterpiece
Interested in learning how to write a book and send it to children's book publishers? Come on over to The CBI Clubhouse for audios, videos, insider writing tips and much, much more!