Why would a nanny need breaks during the working day? It isn't usual. I think a nanny would be expected to opt out of Regulation 12 of Working Time Regulations in addition to Regulations already opted out of due to Domestic Service in Regulation 19. /link{http://www.opsi.gov.uk/si/si1998/19981833.htm#19\Working Time Regulations 1998}
As their employer the thing to do is to pay for their lunch. I would suggest that nanny eats lunch with the children.
You raise an interesting point with this... as what happens if the nanny won't opt out?
I'm not a lawyer - I'm a nanny, so would suggest seeking legal advise if it was necessary.
However, I expect there is something somewhere which covers this situation, as if the nanny is at home with the children alone, they can not leave those children unattended, so they can take their rest break.
With regard to babysitting as an addition to usual hours, then yes, you can add that to the contract. However if you don't use the babysitting hours that week, then I do not feel you can carry them over to another week, as the nanny was available to work those hours - you just decided not to use them.
As you have a late start on the day you want the babysitting and due to it being the same day each week, I'd just put the working hours down in the contract.
For example, lets say:
Mon 9am-6pm, Tue 9am-6pm, Wed 1pm-10pm, Thur 9am-6pm, Fri 9am-6pm.