I'm sorry you lost one of your girls. The heat can be brutal. I wouldn't bring them inside, makes it worse when they go back outside. They have to be able to handle it. Deep shade and cool water to wade in are best. Cool treats during the hot part of the day like cucumbers and watermelon, not necessarily cold from the fridge but room temp is fine. Also, Sav A Chick a few times a week is helpful. Consider a different breed of chicken, something without a bunch of fluff if that's what you've got.
Good luck!