Sometimes when you divide up your WordPress content into categories it all ends up in sub-categories of one or two main categories that don’t have any content themselves.
With the widget you can add a list of sub-categories even if the parent doesn’t have posts.
You can display and filter sub-categories in various ways:
<ul>
<li>Use the parent category as the widget title</li>
<li>Show post counts in bracket next to the name</li>
<li>Hide empty sub-categories</li>
<li>Add a link to the parent category to the widget title</li>
<li>Show the full sub-category tree so it include sub-sub categories and so on as well</li>
<li>Display the list as dropdown rather than as links</li>
<li>Use the first category of the current post as the parent</li>
<li>Exclude one or more sub-categories from the list</li>
<li>List the categories in reverse order</li>
</ul>

Sub Categories Widget

This Widget lists the sub-categories for a given category.

drmen8x

Show category and subcategory and post in these categories
Please see screenshot to know more…
Features:
<ul>
<li>Show the parent category and subcategory</li>
<li>Show posts in these categories like vnexpress.net</li>
<li>You can limit post to show</li>
<li>You can show or hide subcategory</li>
</ul>

VNEcategory

Show category and subcategory and post in these categories

Sub Categories Widget vs VNEcategory