I think if the school wanted to evaluate how you'd cover a complex topic, they'd allot more than 15 minutes for your demo lesson
If I were in your place, I'd choose one of the simpler themes you suggested. Teaching comparatives might also be a fun. Whichever topic you go with, just aim to present it clearly and choose one or two brief activities to get the "students" involved. Just pick something you like to teach that would be appropriate for the students at this school.
I only ever had to do one teaching demo. I don't remember how long it was or what topic I had to teach, but I do remember one horrific detail: one of the panel members interviewing me, the school director no less, slouched and kept her chin rested on her arm during the entire lesson. I'm not sure if she was acting like a disgruntled teenager because that's who my true students would have been and she wanted to see how I'd engage them, or if she was truly bored out of her mind. Anyway, I'm sure it threw me off.
The general advice I'd give would be to just practice your lesson several times beforehand so you're very comfortable with your lesson plan and can deliver it as naturally as you would in a real teaching situation.
I hope everything goes well!