Apparently I really am a domestic servant

I have been asked to, essentially, play wait staff for a dinner party the Parents are having next Monday.  I said yes, caught off guard, but the more I thought about it I was quite a bit offended.  I asked for clarification of my "duties" last night and was told that I was expected to clear the table, bring out food, etc. for them and their friends, and that this party would probably not start until 9 or so, lasting all night.  I told her I am willing to help, but that serving food is uncomfortable and a little too close to a domestic servant for my taste, and she just said "ok ok ok ok" through the whole conversation, but apparently I am still expected to be there for the whole dinner party, pretty much manning the kitchen.  I'm really not satisfied with this though.  So great.  I'm a maid.

This is not a new thing, either.  I contacted Rosie, the girl before me, who said that not only was she asked to do this several times, but so was the girl before her.  Rosie didn't like it either, but never said anything.

Here are my problems with this:

1. I already had plans Monday.  She gave me advanced notice that I would be babysitting Saturday, and that I would have to work all day Wednesday and late Thursday since she'll be out of town.  I had assumed that since she gave me advanced notice about Wednesday, Monday was free.
2. I'm already working 10 hours Wednesday (double my usual per-day amount) since she'll be out of town.
3. THIS IS NOT MY JOB.  I am here to take care of her children.  This dinner literally has NOTHING to do with the children.  I am not here to be a maid or a waiter (which I have done, and I got paid a lot more than this to do it).  This was not mentioned in the emails we exchanged before hand, or in the contract we both signed, and if it had been, I probably would not have taken this job.

So since I told her I would do this, and keeping promises is very important to me, I'm going to man this event and suck it up.  Afterwards, though, I think I will have a talk with her and pretty much say "please don't ask me to do this again."  This is disrespectful to me, it is outside the scope of my job and my hours, and I am hurt and insulted that she is failing to respect the agreement we had before I came.  Hopefully she'll get the point.  And if she pulls the whole "the other girls did it" thing, I now have proof that the other girl hated it too.

And hey, we all know this hasn't exactly been my dream job.  If I get fired and have to come home, fine.  Good riddance to you and your devil children.

Maybe I should try to prove a point.  How offensive would it be to come in blackface?  Or maybe dressed like this:

That might be a good look for me.