show child selections on the category overview page

[Greg-Baud]

In the tutorials page  http://docs.virtuemart.net/tutorials/product-creation/186-derived-products-product-pattern-child-products.html there is a description on how to create stockable variants in VM3.
It also refers to a demo page http://demo.virtuemart.net/virtuemart-views/categories-view/product-w-child-variant-detail
I got the same thing working, only on my test site the stockable variants can only be selected on the product page, not on the product overview page, which is a menu option showing a VM category. http://www.greg-baud.nl/Test3.3/index.php/gb-tickets/1/tickets.html
it shows a box "Choose a variant" in stead of the real selection.

How do I enable the child selection directly on the category overview page?

Wim