fix order during export and linking, fixes #2959 and #1361

Closes #2959 and #1361

Merge request studip/studip!1991

lib/models/Courseware/StructuralElement.php

@@ -982,9 +982,7 @@ SQL;
 
     private function linkChildren(User $user, StructuralElement $newElement): void
     {
-        $children = self::findBySQL('parent_id = ?', [$this->id]);
-
-        foreach ($children as $child) {
+        foreach ($this->children as $child) {
             $child->link($user, $newElement);
         }
     }
@@ -1041,9 +1039,8 @@ SQL;
 
     private function getChildrenPdfExport(int $depth, bool $with_children, $user, $doc)
     {
-        $children = self::findBySQL('parent_id = ?', [$this->id]);
         $depth++;
-        foreach ($children as $child) {
+        foreach ($this->children as $child) {
             $child->getElementPdfExport($depth, $with_children, $user, $doc);
         }
     }