500 Error

    $q = $modx->newQuery(
        'modResource',         // <-- 'modResource' automatically goes to modx_site_content
        array(
            'parent'=>13         // <-- A simple array list of fields => values
        )
    );
    $pages = $modx->getCollection('modResource', $q);  // <-- This is the way to get a list of resources

    $output = '';
    foreach ($pages as $res)   // <-- iterates through the array automatically
    {    $output .= '<p>' . $res->getContent() . '</p>';   //<-- getContent() is a nice shortcut
    }
    return $output;    // <-- echo should not be used.

<?php
$parent = $modx->getObject('modResource', 13);
$pages = $parent->getMany('Children');
 
$output = '';
foreach ($pages as $res)  { 
     $output .= '<p>' . $res->getContent() . '</p>'; 
}
return $output; 

$created = $res->get('createdon');
$published = $res->get('published');
$content = $res->get('content'); // it will work with content too.