Check if current page is a descendant of the specified one.

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
 * @link https://gist.github.com/broskees/8fd20d3167c4e7dd2941aaef5e7068cc
 * @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){
    global $post;

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

    if ($page = get_page_by_path($page_path)) {
        return in_array(
            $page->ID,
            get_post_ancestors($post->ID)
        );
    }

    return false;   
}
?>

PSR-12 version