You see bloo, I think you were fine in the second example, but I think you overstepped in the first. It isn't your place to make someone else's kid work - don't we tell people on this site that they shouldn't allow themselves to get voluntold to do things? If his parents wanted him to work, they could have insisted, but you making some kid help set up your party because you don't think it's right for the Grandpa to volunteer to help while the kid doesn't isn't right in my eyes. I say this as an adult who was forced to do work for my cousin's party until I finally just got up and left - I never agreed to help & I don't see why my parents saying they will help means I have to as well.
You're absolutely right and I would totally do it again!
I'll admit I do not like her kids because of their consistent behavior / attitude. In this situation, they brought a child onto my property and then didn't supervise him
(hence taking off with one of our bikes without permission and riding it all over the property and to the neighbor's as well as pestering my DS to quit helping so he'd play with him). A world apart from an adult being voluntold to help, IMO.