Freelance editors can be of most help to your manuscript at a later point in its development. Thus, before you decide to send it to an editor, there are a few (free or relatively inexpensive) things it would be best for you to have done first:

• Joined and attended your local SCBWI events
• Explored Harold Underdown’s The Purple Crayon, which has a lot of great advice for writers and a lot of great information on the publishing industry
• Explored the SFWA site if you’re writing science fiction or fantasy
• Joined a writing group and gotten feedback on your finished manuscript from trusted readers
• Revised on your own
• Explored my blog and that of other editors to be sure you know what editors are looking for
• If self-publishing, examined all the roles you’ll be taking on—and counted the cost of the money involved to edit, design, print, advertise, market, and distribute your book
• Checked out advice from authors and editors in the children’s literature blogosphere (see links here and on my Livejournal mirror site, http://slwhitman.livejournal.com)
• Know what kinds of services you’ll be asking for: developmental editing, line editing, copyediting, or proofreading. There are levels of depth that an editor can’t provide in one go, because the changes you might make on the developmental level will affect what is left/added when you line edit. Same for line editing with later stages.

Once you decide that you’ve done all you can do on your own and with the help of trusted readers, then if you decide to approach an editor, you will get more out of the arrangement. You should feel like you’re getting your money’s worth, as engaging the services of an editor can be quite expensive, especially compared to all the great resources out there.

Once you’ve done all that, then you will feel more confident approaching editors, knowing exactly the services you’re looking for and knowing that your manuscript is in the place it should be for them to be able to help you at the level you desire.