WordPress is_child by slug (add to functions.php)

is_child.php

// Is child
function is_child($post_slug) { 
	global $post; 
  $parent_id   = $post->post_parent;
  $post_slug  = get_post_field( 'post_name', $parent_id );
	if( is_page() && ($post_slug) ) {
        return true;
	} else { 
        return false; 
	}
}