drupal gridview drop down menu - showing all menu and sub menus under one parent
Maybe you could use the Superfish module? Here is a quote about it (from the module's project page):

... integrates jQuery Superfish plugin with your Drupal menus:

Unobtrusive JavaScript.
Keyboard-accessible, screen reader friendly.
Multi-column sub-menus. (Megamenus)
Easing effects with the jQuery Easing plug-in.
Automatic width calculation (sfAutomaticWidth plugin) (Use the development release)
Automatic sub-menu width calculation (Supersubs plugin), automatic sub-menu placement (Supposition plugin; prevents sub-menus from appearing off-screen.)
Touch-screen compatible. (almost)
Small-screen compatible. ( and vertical accordions)
Window width (client-side) and UA string (client-side and server-side) detection for touch and small screen plugins (no CSS3 media-queries)
RTL (Right-to-left) language support.
Plenty of built-in options for designers.
And a whole lot more...