is-descendent-of.php

<?php
/**
 * Is the current page a descendent of page identified by the path (not slug).
 * A page is not a descendent of itself!
 * @link http://wordpress.stackexchange.com/questions/101213/does-is-child-exist-in-wp-3-5-1
 * @param string $page_path The path of the page, e.g. mypage/mysubpage 
 * @return boolean True if current page is a descednent of specified $page_path. False otherwise.
 */
function is_descendent_of( $page_path ){

    if( !is_page() ){
        return false;
    }

    $ancestors = get_post_ancestors( get_queried_object_id() );
    $page = get_page_by_path( $page_path );
    if( $page )
        return in_array( $page->ID, $ancestors );
    return false;   
}
?>