When I was a nanny in college I had the family want to take me with the to Cancun for spring break. They would have paid for it all - but I wouldn't get paid. They played it as "a free vacation"
I heard it as "Come along and watch our kids for free" it wasn't going to be my vacation as I was chasing their 3 kids around the beach. And sit in their room at night so mom and dad could go out to eat.
Now as an adult I understand more of where they were coming from - but just wanted to give an "au pair's" interpretation of such offers.
That's not an option under the agreement you sign with the au pair agency. If she is working then you must pay for the trip and you must also pay her salary for the week. If she pays her own way and uses her vacation time to come along (and not watch the kids or perform any other work duties, like doing the children's laundry), you must also pay her (vacation pay). It's not an option to bring her along and not pay the ~$150 weekly paycheck that they receive.