We have a shop on Joomla J3.8 / Virtuemart 3.2.4.
Now we have the latest version of GK Storebox 3.21.4 and the child product is not shown in the product detail template.
In the original template that was for J2.5.x and VM 2, it was ok just thanks to the $ product_children mentioned below.
It seems to me that in /templates/gk_storebox/html/com_virtuemart/productdetails/default.php missing:
$ product_children = MultipleCart::getProductChildren ($ this->product->virtuemart_product_id, $ product_model);
or something similar...