jeff.cave

This plugin provides a “Sub Page” widget. It lists all of the pages which are children of the current page. If there are no children, it will not display.
Options include:
<ul>
<li>title (obviously)</li>
<li>“No Sub-pages” text instead of blank (configurable)</li>
<li>hide widget when no sub-pages</li>
<li>show siblings when no sub-pages</li>
</ul>
While this is a small widget, I have attempted to make this as configurable as possible, with clean and efficient code (this is a small widget, and shouldn’t slow your site down).

PS Sub-pages

Shows the child pages of the current page. Configurable title, show siblings (or not), and "no child" text

hildende

With installing this plugin you add functionality to <code>wp_nav_menu()</code> to show 
only children of the current page or start with a specific navigation level.

Child Navigation

Adds support for child navigation to wp_nav_menu()

PS Sub-pages vs Child Navigation