dainius.kaupaitis

When navigating on pages widget shows links to child pages. If there are no child pages – widget shows „brother“ pages. 
Link of currently viewed page can be styled through CSS. Widget shows nothing if user is browsing Posts (not Pages). 
Admin can configure:
<ul>
<li>widget title</li>
<li>which pages must be excluded</li>
</ul>
Widget can be localized.

DKA Child pages widget

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

DKA Child pages widget vs PS Sub-pages