make menu stay highlighted when entering child page

Hey, I have this page at https://www.henrybowers.com and I’m using the .current-menu class to make the current selected page stay highlighted in the menu. However, if you go to the shop and continue to the checkout the menu looses the highlight. This is because the checkout is a child page of the shop page. By the way, I’m using the WP e-Commerce Plugin for the shop.

So, my question is, is it possible to make wordpress to think that a subpage is still the current page somehow.

I have to say, that I’m not in any way professional with this web page building thing, I just learning as I go along, to please bare with me if this question seems stupid (and that it’s probably been asked before, I just didn’t know how to search for it…)

/oscar, sweden