Hi Scoopsmom, I live in the DFW area & have raised chickens in Texas for years. Our heat index was 105 degrees today at my house. My girls are around the same age as yours & were also panting & less active. This morning, I watered the grass & bushes where the girls like to hang out & dust bath. After scratching around a bit, there was a lot of laying out in the cool soil with legs stretched out like playboy models. I don't have a misting water system, but I do have a waterhose that has a split in it & sprays up in the air making a mist that the wind blew over toward the chicks but not so much that they protest. I emptied their water & replaced it with fresh cool water every few hours & if they moved spots, I moved the water over to them. Like you, I also gave them cold watermelon. On Pinterest, I recently saw some chicken treats for summer you make in an ice block. I haven't tried them yet, but looks like a good idea. When they went into the coop this evening, it was still really hot. I'm glad my coop is well ventilated!
Hope this helps! 
