Probably they won't have suffered at all. They would huddle together to keep each other warm as the temp dropped. It's not like it got down to freezing.
But I commend your valiant efforts at being a good chick papa!
Every set of chicks will differ in their heat needs. My last batch didn't like it any warmer than 85 their first week. Thermometers are more trouble than they're worth. The chicks' behavior is the best gauge of the proper temperature.