Simple way to add dropdown arrows to a dropdown containing menu?
I would recommend that you use the settings of the Drupal Superfish menu itself. There is an option to use "Auto-arrows".

They can be further themed via CSS.

Superfish settings (in configure superfish block)


This is your site html source. That auto arrow element is there, but because it uses a white background-image you will not see it, so style it like you need it.