Hi there, shamrock838.
Most picture books are not for the middle grade market, so I'm a little confused by your question.
I can tell you that I'm writing an article on nonfiction books for kids right now, for which I have interviewed some editors (and writers). Most say that for a nonfiction picture book, especially for an author they have not yet worked with, they would want to see the whole book, not a proposal.
Are you planning to illustrate your own book? If so, I have no idea how you submit that. If not, you pretty much attend only to the text.
Sorry not to have more/better answers.... Anyone else?