If you are an established author but don’t currently have an agent, be careful how you make your next move to advance your career. If you have completed a top-quality manuscript that you think could be published by the big houses that require agented submissions, you will have to have an agent to get it where it belongs. Go ahead and submit your manuscript to potential agents at this time, but resist the urge to also submit it to various publishers.
An agent might not want to represent a manuscript that has already been seen by too many editors’ eyes. If an agent believes in your manuscript enough to represent you, she will want to take your manuscript straight to the top as a fresh, new voice in the children’s market. She’ll want to give the top editors she works with the first chance at seeing your work and not have to tell them that numerous editors at smaller houses have already seen it (or perhaps even rejected it).