Use the example (see below) before with the $key, that last one does not work.
<?php
$subaktuelles = $site->index()->findBy("uid", "aktuell")->children()->visible();
$neuigkeiten = new Structure();
$key = 0;
foreach($subaktuelles as $p) {
foreach($p->news()->toStructure() as $item) {
$neuigkeiten->append($key, $item);
$key++;
}
}
foreach ($neuigkeiten->sortBy('date', 'desc') as $neuigkeit): ?>