'um, she has had a fair few months to anticipate this, though, surely?
has she not ever spoken about it before?'
'It's not like she has suddenly come down to work one morning and - surprise! there's a baby! - she should have been thinking this through before now'
I think you're missing the point! She has had many months knowing a baby was coming. You're quite correct she did not come down one morning and discover a baby! What did happen was at around 6 weeks they stopped taking the baby with them when they went out and left it for her to deal with. AFAIK with no warning/discussion, just 'we're off out, baby needs seeing to, back by ten, byyeee'. A couple of people here have been surprised that they're out twice a week when they have an 8 week old baby. I think my friend was also surprised, which tbh is fair enough really! So, no she probably didn't think it through/speak to them about it in advance because she didn't forsee it happening.
'it is a sudden increase in duties, but then the addition of another baby was always going to be that, wasn't it, at some point? once the MB goes back to work, there will be an additional child to look after all day too'
I must stress she has no problem with there being a 4th child. No problem with MB going back to work when she is 6/9/12 mths old. The problem is very specifically her evenings being consumed by an unhappy baby.
'quite an increase in work, and there need not necessarily be an increase in pay - nannies are paid per household, not per child'
Again true. I think the addition of 4th child would probably lead to a reduction in her duties in some other area though, if not a pay rise. A pay rise does not always follow the birth of another child because things can be re-worked so that overall amount of work stays the same. If there is a big increase in workload there should be an increase in pay (IMO at least).
'The babysitting thing is a real red herring though. anything could have happened over the last 5 years to interrupt her peaceful evenings - one of the children could have developed a fear of the dark, or sleepwalking, or anything - would she have wanted extra pay then?'
I don't know if she would tbh. I would though. If one of the children developed any issue that meant the 'keep an ear out' I was told at the interview became '4-6 hours a week extra work' I would ask for a rework of the contract (possibly changing hours to 7am-9pm plus 2 nights babysitting). Although sleepwalking comes under the category of 'keep an ear out' and switching on a light for the scared of the dark kid is hardly hard work!