Check if we are on a child or grandchild page of the page.

wp_is_child.php

<?php
/**
 *  Check if we are on a child or grandchild page of the page.
 *
 *  @param   integer  $id      The ID of the page we're looking for pages underneath
 *  @param   boolean  $parent  Whether to check if we are on the parent page as well
 *  @return  boolean
 */
function wp_is_child( $id=0, $parent=false ) {
    global $post;
    if( is_page() ) {
        if( $parent && is_page( $id ) ) {
            return true; // On the parent page
        } else {
            $parents = get_post_ancestors( $post->ID );
            foreach( $parents as $page_id ){
                if( $page_id == $id ){
                    return true; // At a sub page of $id
                }
            }
        }
    }

    return false;
}