This Widget lists the sub-categories for a given category.
Installing is pretty easy takes only a minute or two.
Upload 'sub-category-widget' directory to your '/wp-content/plugins/' directory.
Activate the plugin through the 'Plugins' screen in WordPress.
On the 'Widgets' sub-menu of 'Appearance' you will find a new widget type called 'Sub Category'.
Add one or more of these to your themes widget display areas.
For each widget you add, select a title and category, then decide if you'd like empty categories and post counts shown.
Save your settings.
That's it. Enjoy!