public function addChild(EntityInterface $child, $position = null, $returnChild = false)
    {
        if ($this->exists) {
            if (is_null($position)) {
                $position = $this->getNextAfterLastPosition($this->getKey());
            }

            $child->moveTo($position, $this);
        }

        return ($returnChild === true ? $child : $this);
    }