I think it can depend on age too, mine needed help cooling down at 6 weeks in 100F. I brought them inside when one of them started drooping his wings slightly to cool off.
They seem to display similar behaviors to full-sized chickens who are overheating so just look out for anything like that. Constant panting/drooping wings. Some people give their chicks electrolyte water in heat like that too to help assist them.
Poor girls, I definitely can relate. I feel so bad for our big girls out in the barn. It's gotta be up to 110F sometimes out there, nearly all day for the last 60 days (at least over 100).
Here is what I do to keep them cool:
-Fresh tray of water every morning for them to wade in (18" tray, ~4" deep)
-2 frozen 1gal containers of water on the ground that they can sit around and stand on (sometimes I swap them out for fresh ones halfway through the day)
-Frozen water treat "pucks" up to 2x a day (shallow container of refreshing fruits/veggies like watermelon, blueberries, strawberries, peaches, any scraps around the house-- cover with water and freeze. Crack it up into a few chunks and spread it around the coop so everyone has a chance). I always have about 6 of these ready to go in the freezer.
-No corn in the scratch
-Hose down the area near the coop with water to help cool the air down, I also hose down the barn walls (which are sheet metal) and the interior of the coop so that their feet get wet. It also helps keep the ground cool so that when they sunbathe there is a cooler layer beneath the surface. 1-2 times a day.
I used to use a big box fan, and I would still consider it for temps in the 90's, but when it's over 100F it feels like the box fan just turns the barn into a dehydrator. The air is too hot, so I dont use it when it's so hot. They also usually have a breeze, so that's helpful.