We have a nanny because my wife and I both work, and I agree with the other posters. Despite it being at Disney World, this is a work trip for you, you should be paid for it. I would never consider asking our nanny to come and take care of our kids and not pay her expenses as well as her salary for work. Possible exception would be if this is a close friend or family member and you are trying to do them a favor, but you shouldn't feel obligated to do so.
IMO:
Non-negotiable - cover your flights, hotel, and meals (amount of $/day for food should be agreed upon prior)
Negotiable:
What are your expected duties, and based on that how many hours are considered "work hours" that are paid per day?
You should probably know going in what you are going to be doing. Just watching the kids at night so the parents can have a nice dinner alone? Going with them to the parks to help them chase after Little Timmy? What about nighttime? Based on that, you can decide how many "hours" that is worth and how much you should get paid per day.
To be honest, I would probably just pay full work days, no matter what the expected duties are. Imagine they weren't going to Disney World, but instead they were going to BoringMcBoringsville USA to visit the grave of their dead cat. They just wanted you there to watch the kids for 2 hours so the parents can attend Mr. Meowington's' memorial service, but you could have the rest of the time off. Sure, you are only "working" for 2 hours, but at the same time you are spending your "time off" touring the National Museum of Dried Paint and eating at Discount Dan's Day-old Diner. Is it really fair to only pay you for 2 hours? From a business perspective, Disney World is no different than BoringMcBoringsville, as there is no guarantee that an employee actually wants to go to Disney World or will find it enjoyable. People take business trips all the time to Hawaii, Las Vegas, etc and they generally get paid for it. However, I could see an argument for only paying the actual hours you are working if you are agreeable to it, just something to work out with the parents.
Park admission?
If you are not expected to go with them to the parks, then it would be reasonable for them not to pay for your park admission, but if you are expected to help take care of the kids at MK, then they should pay for your admission.
No matter what you decide, I think it's important to have an agreement ironed out ahead of time so that there are no hard feelings during the trip.