I'm going to shout at my uncle and threaten to call the cops because of the way his kids treated my cat.
What did his kids do to your cat?
My cat pooped on the floor so they teased it.
It doesn't sound like the kids did anything particularly bad here. It's still wrong of them to tease animals, but it's best just to calmly talk to your uncle about this so he can deal with his kids' behavior. Calling the cops over this is very unnecessary.
I'm not going to sit here and just let them tease my cat by laughing at it!
While it is rude to laugh at an animal and to tease it, yelling at them will just make the situation worse. It will escalate the confrontation and make the kids scared of you and your uncle mad at you. If the kids won't behave, you can always hide the cat in a room until the kids are gone.