Check if page is a child of, takes a page ID

is-child.php

/**
 * Check if a a page is child of a given ID
 * 
 * @link(_blank, http://bavotasan.com/2011/is_child-conditional-function-for-wordpress/)
 * @param  int $page_id The parent page id to check
 * @return bool
 */
function n26_is_child($page_id) {
    global $post;
    if (is_page() && ($post->post_parent == $page_id)) {
        return true;
    } else {
        return false;
    }
}