Add Custom ID to Lineitem If LineItem is a Variation (option group) of a Parent Lineitem


I need to add a Custom Id to LineItem(s) that are variations (color, size) of a Parent Line item.
Since Now i tryed this:

use Shopware\Core\Checkout\Cart\Event\BeforeLineItemAddedEvent;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;

class AddDataToPage implements EventSubscriberInterface
    public static function getSubscribedEvents()
        return [BeforeLineItemAddedEvent::class => 'onLineItemAdded'];

     * @param onLineItemAdded $event
     * @throws \Shopware\Core\Checkout\Cart\Exception\InvalidPayloadException
    public function onLineItemAdded(BeforeLineItemAddedEvent $event)
        $lineitems = $event->getLineItem();
        $event->isMerged() == true;
        $payloaditem = $lineitems->getPayload();
        // I'm stuck here i want to do this: if lineitem is a variation add customfield to lineitem with variation and his parent
        if($lineitems->getReferencedId() == "df50709c534e499488c6fe02312ca526"){
            $lineitems->setPayloadValue("myChilds", true);
            $lineitems->setPayloadValue("myChilds", false);

Unfortunately there is nothing Common between Line Item(s) with variations.
Does anyone have an Idea?