Create a Great Introduction: Cover and Query Letters. Alayne has added some great links for further study as well.
Query Letters that Worked at Sub it Club for reference because I know that when I was figuring out how to write queries it really helped me to look at successful queries. The first one listed when you click on the link is by picture book author Rebecca Colby. She went above and beyond and showed how she progressed with query letters, what she did wrong, then breaks down the query she used to obtain her agent and showed us what she did right. It is so helpful! I just love the generosity of writers in the kidlit world. Definitely check it out to figure out what you are saying with your query!