Showing hidden pages on request

Hi everybody,
I can’t figure out whether it’s possible to show hidden subpages in the sidebar on demand. For example I have a mother page called ‘poetry’, and the child pages of this page (which are all poems) are hidden in the sidebar. Is it possible to make them visible when you click ‘poetry’ in the sidebar? So only when you have that motherpage opened, in order to make the navigation more easy?
I’m now using ‘water’theme and the sidebar widget, in which I excluded the specific pages.
I hope I’m making myself clear enough in order for you to be able to answer!
thanks so much in advance.
-Marlies