We always leave ours strictly alone for the whole of the brooding and hatching period. I know many folk regularly lift them off the eggs to eat, drink and dung but I take the view that it's not in Mother Nature's interest to let a broody starve and, indeed, it's never happened to one of mine yet. Similarly, I just chalk the hatch date on the broody coop and keep an eye on her around then and a couple of days before change her feed for chick crumbs and her water container for one suitable for chicks. As soon as she brings the chicks outside I take the opportunity to clean out muck, eggshells and unhatched eggs from the coop then again leave her to get on with it. I've heard of hens turning on their chicks and kilingl them if stressed so I do everything I can to avoid that.