We'll create fresh WordPress site with Add Descendants As Submenu Items installed. You have 20 minutes to test the plugin after that site we'll be deleted.
When adding a page, category, or any hierarchical custom post type or taxonomy to your navigation menu (Appearance → Menus), this plugin allows you to optionally automatically add all descendants (children) of that menu item as submenu items.
In short, you’ll no longer have to manually maintain submenus when you add a new child page to your site.