How can I show all products in a top level category sorting by the child categories?

Question

I have this category structure below:

When I access Men page all products of child categories are there, but they are showing in a random mode, i believe. I want to show the products in the main category (Men) sorted by child categories order.

Product Collection function:

public function getProductCollection()
    {
        $current_category = $this->_registry->registry('current_category');//get current category
        $collection = $this->_productCollectionFactory->create();
        $collection->addAttributeToFilter('visibility', \Magento\Catalog\Model\Product\Visibility::VISIBILITY_BOTH);
        $collection->addAttributeToFilter('status',\Magento\Catalog\Model\Product\Attribute\Source\Status::STATUS_ENABLED);
        $collection->addCategoryFilter($current_category);
        $collection->setPageSize(999);
        $collection->addAttributeToSelect('*');
        return $collection;
    }