I spent the day figuring out query letters for agents. I've been sitting on my story, The Emerald Ring, for about a year. The economy was so bad I didn't want to sent out my manuscript when rejections abounded.
It's really important to do your homework before you send out your query,
whether for agents or editors. Agents/Editors are so busy they appreciate a little time spent on your part as to whether they'd even be a good match. I like to check out blogs and websites. Often times an agent will come right out and say what they like and don't like. I'm always looking for people who like fantasy. Find the agent/editors out there who like your genres, who like kid lit, who are looking for submissions.
Next, check out their submission guidelines. Today I queried Michael Sterns at Upstart Crow Literary. He likes the first 20 pages of a manuscript sent and only takes email submissions. I also queried Jennifer Jackson at Donald Mass. She doesn't mind snail mail and requests only the first 5 pages of a story. There are other agents who request a synopsis with your query. It's important to check out guidelines. It shows great respect for the agent/editors time.