<?php
if(is_category()) {

	$breakpoint = 0;
	$thiscat = get_term( get_query_var('cat') , 'category' );
	$subcategories = get_terms( 'category' , 'parent='.get_query_var('cat') );

	if(empty($subcategories) && $thiscat->parent != 0) {
		$subcategories = get_terms( 'category' , 'parent='.$thiscat->parent.'' );
	}

	$items='';
	if(!empty($subcategories)) {
		foreach($subcategories as $subcat) {
			if($thiscat->term_id == $subcat->term_id) $current = ' current-cat'; else $current = '';
			$items .= '
			<li class="cat-item cat-item-'.$subcat->term_id.$current.'">
				<a href="'.get_category_link( $subcat->term_id ).'" title="'.$subcat->description.'">'.$subcat->name.' ('.$subcat->count.')</a>
			</li>';
		}
		echo "<ul>$items</ul>";
	}
	unset($subcategories,$subcat,$thiscat,$items);
}
?>

http://wordpress.org/support/topic/showing-subcategories-of-a-parent-page-under-subcategory

List subcategories in wordpress

PHP