If you are writing a middle grade or YA novel, then you should complete the manuscript before you start querying. Agents and editors need to know that you can follow through and finish your manuscript. (Think about what would happen if an agent said, "Yes, I'm interested. Send the full manuscript." You'd have to admit that you hadn't finished the manuscript and you would have wasted your time and the agent's.) So, as tempting as it is to start querying before you've finished your manuscript, don't.

Nonfiction is a different story. With nonfiction, you can create a proposal, which usually consists of an introduction, table of contents, annotated chapter outline, 2-3 sample chapters, author bio, and marketing analysis (description of competing books). Agents and editors can make a judgement about the quality and salability of your nonfiction book based on a proposal.

And if you want to know whether your unfinished novel, or even your nonfiction proposal, has the potential to be published, join a writers group where the other writers are as serious as you are about writing, and who regularly read children's books. You'll get the feedback you need from them about the publishing potential of your manuscript.