Find out all you can about the level you're writing for and ask the publisher if they have topics in mind. Read book on the level from that publisher. If there are sentence and word count limits, etc, I find it better to know them up front.
Unlike V, I write to them in the first draft. This doesn't mean I never go over. Some publishers are stricter than others on this. Reading their books will give you an idea if it's okay to have one or two extra words in a sentence once or twice in the book. Of course, my first drafts will still need revision for story. Story comes first. A book without a good story won't be read again, if at all. I've also found that a twist or payoff at the end is very important. With so few words, most of the story is set up. The end needs to count and be fun because story is everything.
Ask the publisher about word counts, vocab lists, and sentence counts. If they don't have a list of topics they're interested in, think about common experiences and how you can twist them up. Also, consider the curriculum for the age reader you're writing for and try not to duplicate what the publisher already has.
I hope this helps. Good luck.