4-nights at Center Parcs is usually Midweek (so arrival Monday, depart Friday).
I expect the plan would be for the nanny to have their own room in the villa - which would work fine, though as with any live-in position getting the children used to nanny being on/off duty could be an issue. However your nanny may be very flexible and not mind helping out as and when needed, rather than strictly during the hours they would normally do.
Some perks the nanny may well like are things like a Spa Sauna session (6pm-9pm slot I tend to like best, it's also the cheapest). Your nanny may well like to do some other activities as well - either on their own or with one of your children. For example Canoeing on the lake, treetreking, zip wire, rock climbing, low ropes (Action Challenge).
Consider how you would be doing food - if self catering all the time, who would be doing the cooking, the shopping etc. I would suggest that nanny gets all food provided for them whilst on and off duty. Extras like alcohol are for the nanny to buy - also that raises a question... would nanny be permitted to drink alcohol when off duty?
Has your nanny been to Center Parcs before? If they have then they will already know what to expect, whereas if they haven't they may be wondering what they have let themselves in for.
Technology wise, make nanny aware that they probably won't get their own TV (or are you offering the nanny the master bedroom? Note: check your villa type for details of what is available as they do vary), that mobile phones often don't work, and WiFi isn't available in the villa (unless you are in Executive).
Consider how your nanny would get to and from the particular Center Parcs you are going to (I'm a frequent visitor to CP Longleat). Would they be travelling with you, or would they make their own way there? Personally when going to help a family I find it better to get there under my own steam... as then I can arrive at whatever time I like prior to 3pm (usual time for villa access). If nanny drives, consider what you will do about mileage payment... would you pay that on top of the physical hours, or would you say as childcare wasn't actually being done during the travel time that you would pay the daily rate for that day (even though that first day is 3pm to say 7pm) and the remaining salary for that day was to compensate for the travel cost and travel time. How far would nanny be travelling... for me CP Longleat is under a 2 hour drive, the CP you are going to may be quite some distance from your home.