function parent_category_template( $template ) {
if ( is_category() && cat_is_ancestor_of( $parent_category_id, get_queried_object_id() ) ) {
$new_template = locate_template( array( 'category-parent.php' ) );
if ( '' != $new_template ) {
return $new_template;
}
}
return $template;
}
add_filter( 'template_include', 'parent_category_template', 99 );