One side of their brooder should be 15 to 20 degrees cooler. You can kill them quick by trapping them under a heat source with no space to get away from it.
I am surprised they are still alive. It is best to do your own research and check 2-3 sources.
Oh also use a $10 dimmer cord on the ceramic heat emitter so I can adjust the temperature output. Also works on red heat lamps just make sure the wattage on your heat bulb doesn't exceed the wattage limit on the cord.
Also have a Zilla thermostat that turns the heat on/off to maintain the temp...
I use a mama hen heating pad with a ceramic heat emitter for supplemental heat. I want it to be as natural for them as possible and a red heat lamp creates too much night time light.