After school nanny - help with contract

[MumToOne2010]

We are employing a nanny largely to do after/before school work but she won't be term time only as we intend to use her for some (not all holiday care) and we also want to include some evening hours for babysitting in her contract. She wants it all paid equally over 12 months. How do we do this. It's likely she will do more hours during school holidays. Do we just make provision for overtime or is there an easier way?

You really need to firm up on the hours if you are to arrange to pay equally over 12 months. At the moment I doubt you could tell me the total number of working hours per year.
Try to work out what the minimum number of hours per week would be for term time, and what they would be during holiday times. Then calculate the total yearly hours.
What will you do about nannies holiday - will they only be able to take that during school holiday times? If the holiday hours are a lot different to term time hours, how will you calculate what is reasonable for the payment when nanny is on holiday - average of past 12 weeks perhaps?
Can you agree upon a certain amount of fixed hours per week, regardless of it being term time or school holidays? Then do that over the full year, split into 12 payments. Then do the rest (as long as it is just a few hours here and there) as overtime, taking account of holiday entitlement building up on that.