A few thoughts. I wouldnt pose them in that place. I understand you need a roof, but the high contrast pattern of the posts against the white sky really distracts from your subjects. Too much competition from the surrounding patterns. Second, use some fill flash. That would brighten up their faces a bit, put a bit of twinkle in their eyes. And lastly. Not only are they not smiling, they arent even touching in all but one of them, not even close to each other in a couple more. Not smiling is tough enough, but if you pose them interacting with each other a bit more, perhaps they wlll respond to one another if not the camera.