If I was a lawyer advising a large company I might say that there is no point getting anything signed about the holiday pay, the ECJ has already shown that it is willing to ride roughshod over the express wishes of freely consenting parties in favour of the exact form of European Directives - in this matter and elsewhere.
But noone is going to take a parent employing a nanny to Europe, and if it ever came to an Employment Tribunal, bending over backwards to ensure that everyone is clear about what is agreed would be looked upon favourably.
So yes, getting her to sign something like this might be a good idea:
'I understand my statutory entitlement to paid holiday in accordance with the Working Time Regulations 1998, and my entitlement under this contract to [14.5] days of holiday per annum. I will take holiday during the weeks of the year that I am not required to attend this employment and wish to be paid for this holiday pro-rata over the [39] weeks of the year that I am required to attend.'
Then just show separately £[14.5 x daily rate / 39] holiday pay on each payslip - subject to Tax and NI of course.
Because this has ended up quite specific, I need to say: please note that everything I have posted in this thread is for information purposes only and does not constitute advice.
Regarding rest breaks, where an adult worker's daily working time is more than six hours, the worker is entitled to a rest break. This is 'an uninterrupted period of not less than 20 minutes'.
You could put in the contract
'Subject to the following paragraph the Nanny shall take one rest break of at least 20 minutes during each day and shall not be required to perform her duties during this rest break.
The Nanny will ensure the safety and well-being of the children at all times when under her care including during any rest break.'
... but unlike holiday arrangements there is no requirement to put this in a contract or Statement of Terms and Conditions so I would leave it out.