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function wpb_list_child_pages() { 

	global $post; 
	
	if ( is_page() && $post->post_parent )
		$childpages = wp_list_pages( 'sort_column=menu_order&title_li=&child_of=' . $post->post_parent . '&echo=0' );
	else
		$childpages = wp_list_pages( 'sort_column=menu_order&title_li=&child_of=' . $post->ID . '&echo=0' );
	
	if ( $childpages ) {
		$string = '<ul>' . $childpages . '</ul>';
	}
	return $string;
}
	
add_shortcode('wpb_childpages', 'wpb_list_child_pages');