Hello,
In this case you need copy a function from the parent theme into the child theme’s functions.php and modify the function to your liking.
I’ve did it for you. You can check the function in function.php file.
Also you can read this article for better understanding how to customize the child theme
https://support.woothemes.com/hc/en-us/articles/203105897-How-to-set-up-and-use-a-child-theme
Regards,
Robert Hall.