'Sorry, would you mind not swearing so much? There are young kids here.' In your politest, least stuck-up tone. Say thanks, smile, and put your headphones back in to avoid chance of a discussion coming up about your audacity. But also... avoid being too judgemental. They're just people too, merely slightly different. In another situation you could be good friends for all you know, I'm sure you've befriended someone similar once who could have been in the same family in the same manner.