Unless you're leaving the doors open for extended periods of time, they should be fine. Think of a momma brooding chicks. At this age, even in the cold, they're out from under her for a good hour at a time. And with a broody, you're either toasty warm under her, or at whatever ambient temperature is, like it or not. No middle ground, no radiant heat circle, etc. Granted, hens don't usually brood chicks this time of year, but it has been done with good results. A large brooder, maybe heat lamps at each end with a cool space in the middle. Or multiple brooders, with that many chicks. I'm not sure you could do heating pads with that many chicks, that would be a lot of pads.