Sorry for your loss. In hot summer weather, I like to put a flat pan of cool water in the shade for chickens to stand in and cool there legs. Electrolytes in the water, keeping the water cool in the shade and using frozen waterbottlescan really help. Fans in the coop, and providing shade or...
I would also consider coccidiosis being a problem as well. The treatment is Corid (amprollium) for 5 days, and treat all chicks. Here is some info: http://www.the-chicken-chick.com/2012/12/coccidiosis-what-backyard-chicken.html