There is Sub Menu feature built-in to the system.
Sub-sub pages are automatically displayed in DHTML menu.
Sometimes Submenu need to be customized, there are 2 type of customization.
When you edit subpage, click on right corner 'Submenu style' link:

You will be redirected to Menu Options page:

You have 3 main options here:
Submenu have some default colors & fonts. You can change it if you wish, to do it, go to 'editor_images' folder and find 'menu.css' file and copy it. New file call 'custom_menu.css'
Now, all you have to do is modify 'custom_menu.css' file