Hello,
your plugin is really great.
I am just wondering if there is a way to show or hide the widget also automatically for a sub-category (automatically same settings as parent category)
Yes, but with a piece of code as described here: http://www.dfactory.co/docs/restrict-widgets-plugin/filters/rw_display_widget/
You can make literaly any combination avaiable in WordPress conditional tags (http://codex.wordpress.org/Conditional_Tags)