Well if they don't know their world geography, you could always say things like they live in the jungle or the mountains. Like pineapple said, demonstrations with live plants would be great. Maybe using a projector to show what you're doing on a larger screen would be better than making them crowd around and squint. For that same reason, it might be better to point stuff out on a large diagram instead on the small plant. I wish they did something like this when I was in preschool!