WordPress: [concept] Cross-CPT Parent/Child

gistfile1.aw

<?php

//Plugin Name: [concept] Cross-CPT Parent/Child
//Description: Just a demo!.

add_action( 'init', 'kl_post_types');
function kl_post_types() {

	register_post_type( 'parent', array( 
		'label' => 'Parent',
		'public' => true,
		'hierarchical' => true,
		'supports' => array( 'title', 'editor' ),
		'taxonomies' => array( 'category' ),
		'rewrite' => true,
	) );

	register_post_type( 'child', array( 
		'label' => 'Child',
		'public' => true,
		'hierarchical' => true,
		'supports' => array( 'title', 'editor' ),
		'taxonomies' => array( 'category' ),
		'rewrite' => array( 'slug' => '' ),
	) );

	add_action( 'add_meta_boxes', 'cross_cpt_parent_box_init' );

}

function cross_cpt_parent_box_init() {
	add_meta_box( 'cpt_parent', 'CPT Parent', 'cross_cpt_parent_box', 'child' );
}
function cross_cpt_parent_box() {
	global $post;
	$parent = isset($post->post_parent) ? $post->post_parent : 0;
	$args = array(
	    'child_of'     => 0,
	    'sort_order'   => 'ASC',
	    'show_option_none' => '(none)',
	    'sort_column'  => 'post_title',
	    'hierarchical' => 1,
	    'post_type' => 'parent',
	    'selected' => $parent,
	    'name' => 'parent_id',
	);
	wp_dropdown_pages( $args );
}

add_filter( 'rewrite_rules_array','kl_rewrite_rules', 9 );
function kl_rewrite_rules( $rules ) {
	$newrules = array();
	$newrules['child/([a-z0-9-]*)/([a-z0-9-]*)$'] = 'index.php?post_type=child&name=$matches[2]';
	return $newrules + $rules;
}

add_filter( 'query_vars','kl_query_vars', 1 );
function kl_query_vars( $vars ) {
    return $vars;
}

add_action( 'wp_loaded','kl_flush_rules' );
function kl_flush_rules(){
	$rules = get_option( 'rewrite_rules' );

	if ( ! isset( $rules['child/([a-z0-9-]*)/([a-z0-9-]*)$'] ) ) {
		global $wp_rewrite;
	   	$wp_rewrite->flush_rules();
	}
}