snippet FirstChildRedirect

gistfile1.txt

$docid = (isset($docid))? $docid: $modx->documentIdentifier;

$children= $modx->getActiveChildren($docid, 'menuindex', 'ASC');
if (!$children === false) {
    $firstChild= $children[0];
    $firstChildUrl= $modx->makeUrl($firstChild['id']);
} else {
    $firstChildUrl= $modx->makeUrl($modx->config['site_start']);
}
return $modx->sendRedirect($firstChildUrl);