[Snippet] OneChildRedirect

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

$children = $modx->getActiveChildren($docid, 'menuindex', 'ASC');
if (is_array($children) && count($children)==1) {
    $firstChildUrl= $modx->makeUrl($children[0]['id']);
    return $modx->sendRedirect($firstChildUrl);
}