If I'm invited to a party, should I be expected to work the party also?

A co-worker invited me to her son's 7th birthday party. I arrived at the appointed time and sat down at the table. The co-worker came over and said "What do you think you're doing?" I replied something to the effect that I was waiting for the party to start. She said "I didn't invite you to ATTEND, you're here to WORK!" Then I spent the next 3 hours, along with another co-worker who had also been duped, serving and cleaning up after about twenty 7-year-olds. I was livid, although I kept it to myself. I just umed the invitation was to be a GUEST, not to work the party. Should " the mom" have been more specific with her invitation? I suppose if she had said "I need help with a bunch of screaming 7-year-old kids at a birthday party," nobody would have attended. How should I have handled it?