Kurwa. The quintessential Polish curse word. Not much of a surprise that the kids would pick it up from their mom...and that she'd be mad at them for using it.
What they got into trouble for in the first place though is anyone's guess...but the cursing probably won't affect how long they're grounded, that particular word is just to be expected, really. XD
Kate's parents probably did the same with her the first time they heard her using it.

Art by  KittyDee