Description
Automatically adds sub pages to the correct menu (if enabled) when the page created/edited and removes pages from their old menu when they should be removed.
Instructions
Just check the “Automatically add new top-level pages to this menu” Checkbox for the selected menu then start creating or editing the subpages. (Appearance>Menus then scroll to menu settings at the bottom)
Menu items should be automatically removed once they no longer belong in the menu, but if they were removed manually from the database etc, you can just use the default menu editor to remove them