We have an au pair and we are bringing her with us to WDW in Dec. We are actually going to the west coast of FL for Thanksgiving week and then to WDW for a week after. We will be gone two weeks. We had told Clara that if she decided to extend for a second year we would pay for her to come to with us to WDW, if she wanted to and that the time would count as vacation. That being said, we are now counting only part of that time as vacation. She has a week of vacation scheduled for the summer b/c her mom was supposed to be flying in (see below). So she will now work 45 hours (probably will wind up less than that) over the 2 weeks we are in FL. Good for us because we can go out to dinner one night alone - or stay in the parks one afternoon while the kids nap, etc.
An au pair is part of your family, she is not a employee. Agencies are very specific about that. We correct her homework for her ESL class - we let her friends sleepover our house - we took her on a special trip to NYC alone with us for Christmas - I get on stub hub and help to find Bon Jovi tickets - she is invited to all family parties, dinners, etc. Thus, if she has decided to go on a family vacation, and winds up taking a kid to the bathroom or handing your child a glass of milk - that does not make it any less of a vacation. If the au pair wants to go on a family vacation and experience wdw, there will be normal family stuff going on, just like at home when she is "off duty" but in your house. That of course is not to say she can't go off alone. If you expect her to be alone with the kids, then those hours are not vacation. We do everything by hours in our house. So, if she watched the kids for 8 hours while you are away - I would only count week as 4 vacation days.
To be honest, we would have done things much differently than we did with hindsight. I would have offered to pay for her hotel and food and had her pay for her park ticket and airfare - or some variation thereof. Of course, I would also not have paid to have her mom fly in over the summer and make arrangements for the kids for that week only to have her mom be irresponsible and wait until now to try to get an appointment with the consulate - and now not be able to get an appointment - thus, throwing a thousand dollars down the drain.
If you are in the same situation as we are, with limited vacation time, if she doesn't want to come, I would request that she take that week as vacation. If she really wants to come to WDW and you would like her to come with you because you like her, maybe make the 1/2 of the week count as vacation. There is no way she will be able to pay for the whole trip on her salary.
We will not be as generous with our next au pair. Clara is our first au pair and we have learned a lot!