How would you react if a child swore?

I'm talking about a child of maybe around 3 years of age. Imagine she swore, using various swear words, how would you react? Also, if you were a preschool teacher of this child, and you heard her swear, what would you think? I have a 3 year old cousin and she is going to go to nursery (in the UK) soon, and she swears like there's no tomorrow. I don't know what the teachers would think of this, her parents and brother are maniacs. She has no manners whatsoever, she spits anywhere and just thinks it's okay to take her top off in public and throw a tantrum. She lives with her neglectful mother who doesn't teacher her anything. And her clothes are hardly ever ironed or clean, she's also stick skinny. What can I do to wake her mother up? Her mum just sits in her room all day on her laptop and phone. The kids are spoiled as well, her 7 year old sister has an iPhone 5 (even I don't have one and I'm 17) :/

I'd beat them to within an inch of their life

She is much too young to fully comprehend what she did is wrong.
The more you react at this age, the more they will do it - just don't do anything to encourage it and tell her swearing is bad.
Now if she were say 7 or 8 then she would have to be spanked, without question. But 3 is much too young.

Well I don't know if there's much you can do since they are not your kids, but sounds like the mom seriously needs to disiplinte her kids a three year old should not be swearing. Does the mom swear a lot at home and it that where she learned all of the words? Also that is really ridiculous having a 7 year old have an iPhone 5! I'm only 14 and I see 3rd graders that have better phones than me! Those kids sound very spoiled and until the mother does anything they will probably just stay like that.