Inside the chicken coop, we made a stand out of cement blocks, leaving a space in the middle. Inside the space, we set a single light bulb that can be plugged into a wall outlet. Then, we set the water container over the light bulb, like this.
We only plug the cord in on very cold days (and nights). The heat generated from the light bulb is just enough to keep the water from freezing. There were a couple of times when there was a very thin layer of ice on the water, but it never froze completely and the girls were still able to get drinks, which they appreciated greatly.