K.N.
Honestly I think it is totally relevant to ask the age of the sitter, and her relationship to you. Is it a teen who is a good friend of the family and who will be happy to get a vacation and lounge by the pool when she doesn't have the kids? Will they be attending the party with you ect, or free to do as they wish until a certain time?
Talk to her, see what she thinks, could be she isn't expecting to be paid much, since it's a free trip. I was a live in nanny for a long time, all my expenses were paid, along with gas money, and I had free child care for my son then while i worked, but i wasn't paid outside that, and still considered myself very lucky, once you broke down rent, utilities, food and childcare, i was making really good money.