I would like to create a custom menu (in this instance it's for the excellent Twenty Ten theme) in which I can check a box that allows a menu item's child pages to automatically be added to the menu.
Child pages are already automatically added in the main menu when it's not a custom menu.
For example: when I create a custom menu and add pages to it, in the menu item options for each page, I think there should be a checkbox that simply says "Automatically add subpages of this item to the menu."
This seems like an obvious functionality to me. I wonder why it's not already there.