Should your PhD students call you by your first name?
Yes, a PhD is essentially an apprenticeship in academic research, so they should be treated as a colleague in potential (it seems normal practice for an RA to refer to their supervisor by their first name).  Also I think it is a bad idea for researchers to be overly formal and deferential towards their supervisors; if you ware working at the cutting edge of your subject, not all of your ideas will be good ones, and the PhD student should feel comfortable pointing out where they feel this is the case.  This sort of self-skepticism (being comfortable with the idea of being wrong occasionally) is a key component of being a good scientist, and it seems to me to be difficult to communicate this by example if the student is constantly reminded of their place in the hierarchy by making them call me "Dr Marsupial".