Split a Custom Menu into parent and child menus

Hello,

I am looking for help, tutorial or documentation on how to split a single custom menu (resources > menus) into a two-tier menu system. From what I have found and tried requires to use wp_list_pages. This defeats the purpose when using the wp_nav_menu option?

I would like to be able to use a custom menu as a top level horizontal menu and then have a vertical child menu that displays only pages of the selected parent. I am also needing teh child menus to drill down (expand/collapse) the deeper you go.

Horizontal Parent Menu:

About | Services | Docs | Partners

Vertical Child Menu:

Services

• Child Page 1
• Child Page 2

• Sub Child 1 (current page)
• Sub Child 2

• Child Page 3

Any help or direction would be much appreciated.