Just to second the suggestion for iChat and point out that iChat has a (command-line enabled) AUTO ANSWER facility. So when you ring your GrandParents they don't even need to click to answer.
I don't find the iChat "Chat Request" panel to be that intuitive (for GrandParents). It just displays a pane with the request annoucement but no buttons. You need to click on the panel to make it expand to show the accept/reject buttons.
Auto-Answer can also help if GrandParents aren't computer literate or have trouble seeing the small on-screen pointer.
Of course, most people don't like the invasion of privacy that this allows but then if you are thinking of video-chatting 24x7 this should not really be a problem.
Google for how to do it
"Though a program be but three lines long, someday it will have to be maintained." -- The Tao of Programming