customize stock level indicator

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Hi I'm using generic child variants for the products with size attribute - so I have one parent (that can also be ordered - has quantity) and a few childs for each product. The stock level indicator in the category view shows only total amount of parent product. I would like it to show parent + childs products amount. Can anyone help me with that?
Regards

my Jommla: 2.5.28
my Virtuemart: 2.6.22