I don't ->get() it!

$output = '';

$wistjedat = $modx->getObject('modResource',21);

$criteria = $modx->newQuery('modResource');
$criteria->where(array(
           'published' => 1,
           ));
$weetjes  = $wistjedat->getMany('Children',$criteria);

$rand = rand(0, count($weetjes)-1);
$hit  = $weetjes[$rand];
$output .= $hit->get('content');

return $output;

$output .= $hit->get('content');

<?php

$output = '';

$c = $modx->newQuery('modResource');
$c->where(array('published' => 1, 'parent' => 21));
if ($weetjes = $modx->getCollection('modResource', $c)) {
    $rand = rand(0, count($weetjes) - 1);
    $hit = $weetjes[$rand];
    $output .= $hit->getContent();
}

return $output;

?>

    $key = array_rand($weetjes);
    $output = $weetjes[$key]->getContent();