I don't see anything wrong with this except that I think it's got a better chance if you are already inside a shop, then 'hi I'm looking for a present for my cousin and she's about your age, only I don't know what girls like, any suggestions?'
It's really important not to do this one girl after another. And avoid any that look like they're in a rush or not on their own. At worst it's practice talking to girls.