WP3 CustomMenus, automatically adding child pages

I am using the WP3 Custom Menus for the menu in my site.

Some of the pages will have subpages that need to be added to the menu on the fly, so for Page A — I need to automatically add all the subpages for A as they get created/and or turned on and off. Is there a way to do this with the Menu? I can se how you would do it for Categories, but how for pages?