As an off duty LEO, I would politely approach the group, and respectfully say something like the following:
"Fellas, hate to interupt - I know you guys are just trying to have a good time but if you could please keep the cussing to a dull roar around the children here, I would really appreciate it."
It is lighthearted, and simple, and allows me to do the mature thing of "doing something" about the problem. If they choose to escelate, I would excuse myself, and go back to minding my own business. I would then call the local PD and have them all cited for DOC-Language, or at least have them identified by local LE, and go to the muni/JP prosecutor, and swear out a class C complaint, reciting my detailed narrative. Then it becomes an arrest warrant
 - also, any citizen can go to the local prosecutor and request to file a class C complaint for something like that.  They might not take the charges, but if you are detailed enough to establish probably cause, then it probably will.
 - also, any citizen can go to the local prosecutor and request to file a class C complaint for something like that.  They might not take the charges, but if you are detailed enough to establish probably cause, then it probably will.



 (Fighting words??)
  (Fighting words??)