Sounds like a horror film!
There is nothing more terrifying in mom-life than having to parent someone else's child. Typically I play the role of "backup mom" aka support person. If that mom says no, I back them up. So that the children see a unified mom-ness. That is a force to reckon with! Or there are times when the parents are off somewhere else at a party and I'm with my son, and their kids. Normally all is fine, but birthday parties are uncharted territory. You are going to be with children you've never met and parents you don't know. This is where it gets dicy.
So let me tell you the tale. Once upon a time I was at a friend's son's birthday party. The birthday boy and three other boys I didn't know were playing with water guns. There are only three water guns, so my son was playing with one of the other boys. They were minding their own business swinging. There I stood with my friend (parent of the other boy on the swing) and her husband chatting away. When suddenly the other boys decide it is a great idea to soak the two boys without squirt guns. First all is well and the two boys are having fun, but soon the are soaking wet from top to bottom and both boys say no.
One boy (that I don't know) squirts them anyways. My son starts crying. The boy runs off and fills his water gun again. My son is trying to get off the swing when the boy returns and goes to squirt him.
I say, "He said no, you can't squirt them anymore."
The boy turns on me and sprays me.
I say, "You need to go tell your mom what you did."
He laughs and climbs the slide and says "no."
I calmly say, "I'm going to count to five and you better go tell your mom what you did, or I will." I count, he stays. I nod my head and say "Okay." I walk up to the house find the parent of the child and tell them what happened, and ask them to go talk to their son.
They are shocked that their son could ever do such a thing, but they are kind and go talk to him about it.
I wander back to following my son around with my friend and her husband when all the boys are playing with the little robot dogs that the birthday boy got. The water-gun kid takes two of the toys and says that my son can't have one. My son looks so heartbreakingly sad. The bday boy offers him one, but this particular little boy takes it and says no again.
At this point I'm over it. I'm done with this kids behavior. I turn to my son and say, "lets go get ice-cream and we will plan a play date with just your friend another day."
My son smiles and runs for the door.
You might ask yourself what is the moral of this story? I might ask myself the same thing. But the moral is, we can't control other peoples' children. I can't put that boy in a timeout. It isn't my house. I don't even know this child or his parents. But I can tell his parents when he's being a bad friend, I can tell them when he is rude to adults and hope that they will care. And I can remove myself and my son from the situation.
What I truly learn in all these types of situations is what I want to teach my son. I want him to be kind, fair and respectful of his friends and their parents. I want him to be a loving person. I want him to share, but not be a push over. And I want him to know it is okay to leave bad situations that make you sad and uncomfortable. So do your best out there to be kind and fair, and make great tiny humans, because the tiny ones grow up to adults.