In the Pages Menu you can now select "New menu section". This allows you to create sub pages under the parent path without having a parent page.
See the 'Sub Pages' navigation item in this site.
Nest pages inside a dropdown menu
On your computer, open a site in new Google Sites.
At the right, click Pages.
Click the page with the menu you want to update.
At the bottom right, hover over Add .
Click New menu section .
Add a menu section name.
You can drag and drop pages into your new menu.
Tip: When you preview your site, you can see your nested pages by hovering over the new menu.