What are the night temperatures?
To help them adjust a little more gradually, you might put them out for the night and bring them in for the hottest few hours in the middle of the day, or put them out for a few hours in the morning but bring them in for the afternoon.
If the "feels like" is based on humidity, it may not matter as much for chickens as it does for people. We sweat, which works better in dry air. They don't sweat.
I agree with
@Finnie: a frozen soda bottle should not do any harm, and it might help, so go ahead and try it. But put it inside the brooder, so they can get as close as they want. Just put is near one end, so they have the option to get away from it. (Same logic as a heat source: if one spot is a very different temperature than the rest of the brooder, give the chicks the option of being in that spot or away from that spot.)