Just wanted to chime in with my experience, since it is a little different from conventional advice:
I was able to get an agent for PBs only, but I signed with her when I had a sale in hand. I got an offer from an editor days after I had subbed (a different manuscript) to agent, and sent her an e-mail with the update (putting OFFER RECEIVED in the subject line, that will get their attention!) Got a call back from agent same day. As we spoke, I mentioned that I was trying my hand at longer stuff (chapter books, MG), thinking this would be a good thing. She said, "Why would you do that? You are great at PBs, don't split your focus!" HUH? That went against everything I had heard about what agents want. I am glad though, because PBs are where my heart is at, and I am happy to focus there. She did want to see several manuscripts, though, and I think if you are going to query agents, you need to have at least 3 strong, marketable stories ready to go.
Good Luck, Carole!