What have you agreed with the nanny?
I was JUST about to write the same thing.
As opposed to FT nannies where any attempt to skip paying taxes or not pay holiday entitlement is, quite rightly, seen as illegal with a PT nanny it depends on what has been agreed. It's not uncommon for the younger ones to juggle it with bar work or the older ones (and I know i'm generalising but YKWIM) with volunteering/church so it's much much more ad hoc.
hope that helps
I could have just written "What Goldhawk Said"!