I agree with above. I work as a nanny myself starting a live in job soon with a new family.
Whilst it hasnt been ruled out of my boyfriend visiting possibly staying (not dicussed really), i havent brought up staying over for the reason I wouldnt want to make my new nanny family uncomfortable, also as mentioned above boyfriends guests are not vetted so again for this reason you have a definate choice not to allow overnight guests.
I have told my new family I will see my boyfriend outside of work hours outside the home once or twice a week where available to go for dinner or whatever and he will go home that night, then I see him at the weekends and this i feel is more approprate/professional and should be the case for others. ( I do get the option to go home however at the weekends and can understand for live in nannies with nowhere else to go may be different circumstances).
At the end of the day you're nanny lives in you're house for free with no bills or costs so you are well within you're rights to set a few ground rules regards guests/visitors this is how i see it as an employee to a family.
susie