code: public function moveChildToPosition(ItemInterface $item, ItemInterface $child, $position)
      {
          $name = $child->getName();
          $order = array_keys($item->getChildren());
  
          $oldPosition = array_search($name, $order);
          unset($order[$oldPosition]);
  
          $order = array_values($order);
  
          array_splice($order, $position, 0, $name);
          $item->reorderChildren($order);
      }
docs: Moves child to specified position. Rearrange other children accordingly.
  
  @param ItemInterface $item
  @param ItemInterface $child    Child to move.
  @param integer       $position Position to move child to.