How to make Dialog go behind MDI child windows

I have an MDI application that has a large dialog that takes up most of the screen on a 640x480 laptop display. The dialog has to be open while the user views the contents of child windows, but the big dialog is always in front of the other windows. How can I make it so that when the user clicks on another window, the window ops into the foreground and the dialog goes behind it? In other words, how can I make the dialog act like another frame window from a visibility standpoint? I thought of putting the dialog content in a CFormView, but then it appears "concave" and doesn't look like a dialog.