How to display only Sub-Categories?

Greetings.  I\'m attempting to have PostGrid create a grid of all the sub-category pages nested inside the parent category.

Under \"Query Posts\", I set up:
<ul>
<li>Post type = Page</li>
<li>Search Parameter = \"Topics\"</li>
</ul>
\"Topics\" is the parent category.

When I view the post grid as a user, only 3 of 30 items appear in the grid.  One of the items is missing the \"featured posts\" image, even though one was assigned.

Am I using the Search Parameter incorrectly?  If I want to create a grid of all the sub-categories under a category, how does one accomplish this?