Most beginner courses cover the following:
present tenses (simple and continuous),
the use of can and must,
comparative and superlative forms of adjectives,
the simple past,
numbers and the alphabet,
vocabulary of jobs, home, likes and dislikes.
I'm making a very sweeping statement off the top of my head but I think I am not far from the truth.
I suggest you find out if the students are absolute beginners (i.e. they have never studied English before) or false beginners meaning that they have some notions of basic English. I'd be very surprised if you are being asked to teach absolute beginners. It is only fair for the interviewers to tell you what the students have been working on recently. It's a necessity at all levels and even more important for beginners.
When you know what they have worked on recently, you can make an informed decision about what to work on. I'd be happy to help when you know what you want to teach. If you can't get any information, the best would be to work on some vocabulary rather that a grammatical item. Don't try to be over-ambitious in what you can do in 20 minutes. I suggest you take a good beginner course book and select an activity from there, using the teacher's book for explanations rather than trying to design your own lesson.