expand/collapse Sidebar category navigation

In my left sidebar i want to create a sort of hierarchial list of categories in the form of:
Parent
Child
Child
Parent
Child
etc.
But heres the catch: When you click on a certain parent category, i want only the child categories underneath it to be visible along with the other parent categories (so basically exclude child categories not associated with the current parent).
I’ve read the documentation on conditional tags and have tried for several days to implement the code to do this but can’t seem to get it right. Any help is appreciated, thanks!
-Jamie