Using the template_include filter in WordPress

functions.php

<?php
add_filter( 'template_include', 'ja_template_include' );
/**
 * Apply a template to all subcategories of a certain parent category.
 *
 * @author Rogier Borst
 * @author Gary Jones
 * @author Jared Atchison
 *
 * @link https://gist.github.com/gists/2787095 Current version
 * @link http://www.jaredatchison.com/2011/10/02/taking-advantage-of-the-template_include-filter/ Original article
 *
 * @param  string $template Existing path to template file
 * @return string           Amended path to template file, or original template path.
 */
function ja_template_include( $template ) {
	if ( ! is_category() )
		return $template;

	$category_info = get_category( get_query_var( 'cat' ) );
	if ( is_wp_error( $category_info ) || 0 == $category_info->parent )
		return $template;

	$parent_cat = get_category( $category_info->parent );

        // Construct a possible file path
        $subcat_template_path = get_stylesheet_directory() . '/subcategory-' . $parent_cat->slug . '.php';

        // Return that path if a readable file exists. Otherwise, return the normal template
	return is_file( $subcat_template_path ) && is_readable( $subcat_template_path ) ? $subcat_template_path : $template;
}