Subcategory madness

subcategory-madness.php

<?php // remove this if you don't need it

function riz_new_subcategory_hierarchy( $path ) {

	$category = get_queried_object();

	$parent_id = $category->category_parent;

	if ( 123456 != $parent_id ) { // Your category ID
		return $path;
	}

	$templates = array();

	// Create replacement $templates array
	$parent = get_category( $parent_id );

	// Current first
	$templates[] = "category-{$category->slug}.php";
	$templates[] = "category-{$category->term_id}.php";

	// Parent second
	$templates[] = "subcategory-{$parent->slug}.php";
	$templates[] = "subcategory-{$parent->term_id}.php";
	$templates[] = "category-{$parent->slug}.php";
	$templates[] = "category-{$parent->term_id}.php";
	$templates[] = 'category.php';

	return locate_template( $templates );
}
add_filter( 'category_template', 'riz_new_subcategory_hierarchy' );