Expanding sub menus in Slide out menu

We love the GP Theme and have a slide out menu for mobile sized screens. I’d like to have the main/parent menu items expand their corresponding sub menus when the menu item word is clicked in the same way that the down arrow on the right expands the sub menu.

Each parent menu item has no link or page; they are simply there for organizing the sub menus.

An example parent menu item would be the “About” menu item. There is no About page so we want the About submenu to expand when “About” is clicked.

Please let me know the best way to make this work.

Thank you in advance.
Bill