Puoi provare con questo codice in category.php:
if (is_category()) {
$this_category = get_category($cat);
if (get_categories('child_of='.$this_category->cat_ID.'&echo=0&title_li=') != "") {
echo "<h1>Subcategories</h1>";
echo "
";
wp_list_categories('orderby=id&show_count=0&title_li=&use_desc_for_title=1&child_of='.$this_category->cat_ID);
echo "
";
}
}