I sense you understand that chickens have a much harder time dealing with heat than cold at any age. Chicks have a special problem in that they lack insulating feathers to help regulate body temperature, whether it's hot or cold in their world.
First, it helps to understand how baby chicks still in down react to temperature extremes. If you've ever watched lizards or snakes on hot days and cold days, you've probably noticed they move to a sun warmed rock or patch of dirt when the day is chilly, and they move to the shade or underground when it gets very hot. This is how baby chicks still in down handle temperature change. It's the most important thing to remember when trying to help chicks cope with temperature extremes.
If you stick a lizard in the hot sun and it can't move out of the sun, it will cook. Same with a baby chick. If you stick a lizard in a freezing spot and it can't move to a warmer spot, it will freeze. Ditto the chick.
In cold weather chicks can regulate their body heat better during the day because they are consuming calories. If the day is cold, they aren't as vulnerable as long as the calorie consumption keeps up with their need for internal heat. But if it's hot, chicks will need a way to shed excess body heat as they absorb it from the hot environment.
I think by now you're getting the idea. You need to provide ways for the chicks to shed excess body heat. I lived in the desert for many years. The average temp where I was, near Yuma, AZ was 117F in the long summer (in the shade). There was a little ground squirrel that had a fascinating strategy for shedding excess body heat. It was always on the move, but from time to time, it would dart into a shadow on the ground where the ground was a bit cooler than the ground (150F or higher) in the sun. It would prostrate itself, like a rug, on the shady spot effectively transferring its body heat to the cooler spot on the ground. Then, refreshed, it was off again.
Give your chicks such a heat exchange mechanism, such as a frozen water bottle to make contact with to exchange their heat into the cool surface. They will know instinctively to do this because their ancestors were giant lizards.