I know the role requirements are slightly different, but when I was working as a Maternity Nanny, I would draw up a list of Maternity Nanny Duties outlining what the client should reasonably expect me to do over and above caring for the newborn. e.g. keeping the baby's room clean, changing table disinfected, doing baby's laundry, preparing bottles, making light lunch for the mother and helping to prepare and evening meal (when living in). The mother and I would agree these terms and sign the sheet which was then put up in the kitchen.
Whilst I would often do far more than "it said on the tin", it was a good way of reminding both parties about the parameters of the role.
When I first started out in this career, one family (who had cottoned on to the fact that I am a trained cook) asked me to prepare a 3 course meal for 13 dinner guests - whilst doing a 24/6 job as a Maternity Nanny! I did it, but after that I bought in the Maternity Duties Sheet for all future bookings!