Heating pad under the chicks would be a very unsafe option. They would have no way of getting away from it. Some folks have used a heating pad successfully by making a frame so it can make a type of cave over them, which would be open on both ends. I would be extremely cautious using one: You'd have to check to be sure it's putting out the right amount of heat, that the chicks can't get stuck under it, that it is approved for moist use, and that it doesn't overheat. It's very easy for a person to get burned by a heating pad, even when it's kept on low... not to mention the damage it could do to a chick.