Force sub pages to have the same template as parent

wp_force_parent_template.php

<?php
/**
 *  Force child pages to have the same template as their parent
 *
 *  @param   boolean  $include_grandchildren  Whether to include grandchildren
 *  @return  void
 */
function wp_force_parent_template( $include_grandchildren=false ) {
  global $post;
    if( is_page() ) {
        if( $include_grandchildren ) {
            // Apply to children and grandchildren
            $ancestors = $post->ancestors;

            if( $ancestors ) {
                $parent_page_template = get_post_meta( end( $ancestors ), '_wp_page_template', true );
                $template = TEMPLATEPATH . "/{$parent_page_template}";
                if( file_exists( $template ) ) {
                    load_template( $template );
                    exit;
                }
            } else {
                return true;
            }
        } else {
            // Apply to just children
            if( $post->post_parent == 0 )
                return true;
            else if( $post->post_parent != $post->ID ) {
                $parent_page_template = get_post_meta( $post->post_parent, '_wp_page_template', true );

                $template = TEMPLATEPATH . "/{$parent_page_template}";
                if( file_exists( $template ) ) {
                    load_template( $template );
                    exit;
                }
            }
        }
    }
}
add_action( 'template_redirect', 'wp_force_parent_template' );