"Y'all" (southern), "you all" (some parts of the Midwest), "you guys" (western), "yous guys" (some parts of the east coast), and a few others I can't think of right now, are all American colloquial methods of expressing a second person plural. While you are correct in that "you" is the technical and universally accepted way of expressing a second person plural in English, the American forms, though not universally accepted, should also be accepted here, as they are expressing the correct thing. In a way, American English is more advanced in this case, as the second person plural is being expressed in a way that is common to most western languages.
Maybe I'm missing your question. You're right, "Hebben jullie dorst?" translates literally as "have you thirst?" which is how one asks "Are you thirsty" in dutch (and spanish and probably others). So asking "Zijn jullie dorst?" would be using the wrong verb (although correctly conjugated!). But I'm a beginner at dutch, so take this with a grain of salt!