<?php
$docs = $modx->getCollection('modResource',array(
    'parent' => 8,
    'template' => 3
));

foreach($docs as $doc)
{
    $pagetitle = $doc->set('show_in_tree',0);
    $doc->save();
    $pagetitle = $doc->get('pagetitle');
    $modx->log(modX::LOG_LEVEL_ERROR, 'pagetitle: '.$pagetitle);
}