The most efficient method would be using the getChildIds() function:
<?php
$children= $modx->getChildIds($documentId, 1);
?>
which will get you an array of child document ids, and I believe it is in menuindex order. You could use this array in various ways to get specific elements. You can do the following to see what is available in the array and how the keys are structured
<?php
return '<pre>' . print_r($children, 1) . '</pre>';
?>
This method does not require a database hit; it uses data that MODx already has available in memory during the request.
MODX Staff