Redirect to first child in wordpress

functions.php

/*if($post has condition) {
redirect_first_child($post->ID);	
}*/

function redirect_first_child($post_id) {
	$pagekids = get_pages("child_of=".$post_id."&sort_column=menu_order");
	if ($pagekids) {
		 $firstchild = $pagekids[0];
		wp_redirect(get_permalink($firstchild->ID));
		exit;
	}
}