Even if she is not your regular nanny, it should not cost her anything to go on YOUR vacation. She is still working. She is not able to be at home with her pals and go hang on the beach or go to a restaurant on her off time away from your family while there.
Come up with a sum that plays off her regular hourly rate, with food stipends (if she wants fancier fare, etc she can pay for it). For example, if you are staying at a resort or decent hotel with a normal sort of restaurant in the lobby or aggreeable room service prices, use those numbers and suggestions for the food stipend amounts when she is "off duty", and to be nice offer a small stipend for an outing for her (pay for the taxi, bus far, or arrange the hotels shuttle service...to the beach, or other notable, safe outing for a single person of her age and awareness of danger). Maybe she will just want to go shopping (spending her own cash, but needing a ride to get there), or to spend a whole afternoon at the beach or hotel pool, umbrella drink in hand (another thing to specify..if she is over 21, some drinking alcohol rules).
Her own room and possibly a bath is nice too...again, if she rooms with the kids, its work ALL THE TIME she is there. Just a thought.
Just remember, this is WORK for her, even if she is going to a fun location. I traveled to ski locations and a Disney park with my nanny family years ago, and each time we learned something new! I got to learn to ski on their dime, but hated sharing a room with the kids (that trip I got a HUGE bonus once home tho!) and in the end they were good experiences as a whole.
Be fair and have a blast!!!