List sub-categories for parent, posts for children

Here’s the behavior that I’m looking for:

When a user views a category (parent) that has children, this displays:

Parent Category Title
Parent Category Description

<a>Child Category Title</a>
Child Category Description

<a>Child Category Title</a>
Child Category Description

<a>Child Category Title</a>
Child Category Description
..etc.

When a category with NO child categories is viewed, then I want this to display:
Category Title
Category Description

<a>Post Title</a>
Post Description

<a>Post Title</a>
Post Description

<a>Post Title</a>
Post Description

Here’s the code that I have for listing Child Categories for a Parent:

$cat = get_query_var('cat');
$categories=get_categories('child_of='.$cat);
  if ($categories) {
    foreach($categories as $category) {
echo '<h2><a href="' . get_category_link( $category->term_id ) . '" title="' . sprintf( __( "View all posts in %s" ), $category->name ) . '" ' . '>' . $category->name.'</a> </h2> ';
			echo '<p> Description:'. $category->description . '</p>';
   }
  }

but then when I go to a category with no sub-categories, it is blank, no posts. Any ideas on how to pull those back in?

THANKS!

[ Please do not bump, that’s not permitted here. ]