Okay, first of all, how many words? If it's a picture book, then the max is approx. 1,300 words. Early reader--short sentences, easy words. Maybe up to 1,000. Early chapter--(about second to fourth grade)--up to about 5,000 words. Middle grade--up to about 15,000 words (maybe 20,000). Young Adult--up to 50,000 words. Those are all ballpark figures. It's also how you treat the subject matter. Do you go into depth? Then that's older kids.
Get hold of either Christian Writer's Market book or the Children's Writers and Illustrators Market. Or go to
http://www.ourcreativespace.com/publinks.html
or
http://www.institutechildrenslit.com
There are other sites that list reputable publishers, too.
I hope this helps somewhat!