This question already has an answer here:
I believe that this plugin will do what you want.
Jamocreations Auto Submenu https://wordpress.org/plugins/auto-submenu/
Here is also a link to the author’s site, http://jamocreations.com/en/artikelen/auto-submenu , where he discusses why he built the plugin and also two other plugins that also work similarly. The advantage of the Jamocreations Auto Submenu plugin though is that if later you want to edit the menu using WordPress Menu editor the entries are actually visible there so you can reorder them or whatever.
The plugin works based on selecting a parent page from the WordPress page editor. If the parent page or parent sub-page has been added to the WordPress menu, then its child pages will automatically be added as well.