I agree that it would be wrong if the grandparents paid the girl right in front of Violets' son, but it sounds like she didn't even get paid that day. My father always gives my kids a couple of dollars every time he see them. Maybe she's the same and this time just used her unpacking boxes as an excuse for what she would have given her anyway.
I'm sorry to say Violets but I don't think SO's parents are in the wrong. It's not right to volunteer to do something and then after the fact expect to be paid for it. Sure it would have been nice if they did throw him a few bucks, but they didn't have to. Now your son knows to always discuss this stuff up front. Lesson learned.