I gave my nanny a bonus when we had the second child - this was to reflect the fact that everything was a bit erratic before and after the baby was born and she was "on call" for part of that time, which did impact on her ability to go ut and get roaring drunk etc (she lives out).
I also gave her a (small) pay rise and we agreed a change in some of her jobs - exactly like someone said earlier, she had more calls on her time and less ability to, eg, do the ironing. When she had a new born and a toddler to look after, I think it was only fair that she also got to have a break when they were both napping, and was not doing the ironing.
I think it is fair to say that her workload pretty much doubled (she acquired a baby and the toddler only went to nursery school two mornings a week at that time). If my workload doubled and there was no recognition of that - whether by pay or by trying to do something about working conditions - then I would be seriously pissed off.
Its really a market thing. I wanted our nanny to stay and it was important to me in the scheme of things to keep the same nanny to give continuity to our toddler and becuase I knew and trusted her and that was very important to me. If I had changed nanny, however, I would have just said - the job involves looking after 2 children - and that would be it.
I liked the point one of the nannies made that you don't exactly fall off your chair with surprise when youngish parents with a small child have another baby!