WenZe

Category Parent Children Selector is a WordPress plugin that helps you in selecting parent-children categories.
Features include:
<ul>
<li>Button to select all categories. </li>
<li>Button to select all categories including children categories. </li>
<li>Button to select first level of children categories only. </li>
<li>Button to select parent and all its children categories under its sub categetory. </li>
<li>Auto select parent categories when a sub category is selected.</li>
</ul>

Category Parent Children Selector

Helps you in selecting parent-children categories.

bobbingwide

Use the Children block ( oik-sb/children ) to list children of the current content as links. 
Works for any hierarchical post type which supports page-attributes, such as page.

SB Children block

List children of the current content as links.

Category Parent Children Selector vs SB Children block