Portfolio Categories: Only display children of CURRENT slug

right now all hirarchical tags from taxonomy “portfolio-categories” will be shown in a ul.

i would like to display only childs from current slug. running out of ideas. any help appreciated!

the taxonomy-portfolio-category.php looks like that:

<?php get_header();
$parentpage = get_template_link("portfolio.php");
$layout = get_post_meta($parentpage->ID, "layout", true);
$terms = get_terms("portfolio-category", "orderby=count&hide_empty=0");
$activeterm = get_term_by( 'slug', get_query_var('term' ), get_query_var( 'taxonomy' ) ); 

$orderby = get_post_meta($parentpage->ID, "orderby", true);
$order = get_post_meta($parentpage->ID, "order", true);

$args = array(
	"post_type" => "portfolio",
	"orderby" => $orderby,
	"order" => $order,
	"post_status" => "publish",
	"showposts" => "-1",
	"tax_query" => array(
		array(
			'taxonomy' => 'portfolio-category',
			'field' => 'slug',
			'terms' => $activeterm->slug
		)
	)
);

$portfolio = new WP_Query($args);
?>
	<?php get_template_part('/functions/page-title'); ?>

	<div id="content" class="contained clearfix">
		<?php // Query Portfolio Categories
		if ( !empty($activeterm) ) : ?>
			<ul class="portfolio-categories">
				<li><a href="<?php  echo get_permalink($parentpage->ID); ?>"><?php _e("Alle", "ocmx"); ?></a></li>
				<?php foreach($terms as $term) :
					$link = get_term_link($term->slug, "portfolio-category");
					$class = "";
					if($activeterm->slug == $term->slug	)
						$class = "class=\"selected\"";
					echo "<li><a href=\"$link\" $class>".$term->name."</a></li>";
				endforeach; ?>
			</ul>

		<?php endif; ?>  

		<ul class="<?php echo $layout ;?> portfolio-list clearfix">
			<?php while ($portfolio->have_posts() ) : $portfolio->the_post();
				get_template_part("functions/portfolio-list"); ?>
			<?php endwhile; ?>
		</ul>
	</div>
<?php get_footer(); ?>