Filter from Categories and possibly sub-categories

Hi,

This is my code I use to filter custom post types:

<!-- Filter per sub-category -->
			<h3>Filter your results</h3>
			<?php
				//
				$this_category = get_category(get_query_var('cat'),false);
				$cat_ID = (int)get_query_var('cat');
				$results = get_categories('&child_of='.$cat_ID.'&hide_empty');
			?>	

			<select name="sub-filter" ONCHANGE="location = this.options[this.selectedIndex].value;">
			<?php
				foreach ($results as $res){
					echo '<option value= "';
					echo network_site_url() . '/category/' . $mylink->slug . '/' . $res->slug ;
					echo '"'. '>';
					echo $res->name;
					echo '</option>';

					var_dump($mylink);
				}
			?>
			</select>

What happens there, I have used the default category listing. The top category takes you to the parents taxonomy menu (www.site.com/category/chocolate).

From the above link, you get a dropdown list with all the chocolates (black chocolate, etc).

However I cannot get the above code to filter some more. How should one go about this?