I think there is a distinction between query letters and cover letters, but most people use them interchangeably. A query letter summarizes your story and asks if you can send it. A cover letter pitches your story and includes the story or whatever number of pages they are asking for. So I would definitely send a brief cover letter describing your story and your bio. But now a days, most of these letters are just your email.
But I checked on the Charlesbridge website and I don't see anywhere where they state they DON'T want a letter. I would definitely check websites before sending anything, info in The Book could be out of date. On the website, they state they want things emailed, so in that email, I would briefly describe your story and why you think it's a good fit for the publisher.