That said, they can indeed overheat if the temperature gets much above that. If you see your baby chicks panting or gasping with their beaks open like the little chick in the photo below, it is too hot in your brooder.
Reduce the heat immediately, and give them a place to escape to a cooler area. Below, you can see that the baby chicks can warm themselves up under the light, cool themselves down on the shady side, or mosey around in between and back and forth. That's exactly what you want, because too hot is just as bad as too cold! You want to make sure your baby chicks are at the RIGHT temperature to be comfortable for them. You don't want them to chill or overheat, as either can be fatal. For that reason, it's important to be particular about your brooder arrangement.
Remember, too, that a brooder getting full sun can get too hot during the day, or a heat lamp too close may not give them a place to cool down. You don't want an A/C vent blowing directly on them either, creating the opposite problem.
And that's the main issue with using a fan to keep them cool: baby chicks need to avoid drafts because they make it difficult for them to maintain their body temperature. Think of *drafts* as breezes that blow directly on your chicks--you don't want drafts. On the other hand, *ventilation* (something they most certainly need in both warm and cold weather) is air that moves above or below your chickens, not at chicken-level. Ventilation is good; drafts are bad.
Down has great insulation value, but it doesn't hold temperature well if there is a draft blowing away the air that the down keeps close to their bodies. If they are plenty warm, a breeze that is also the right temperature is less problematic for them than a chilly one would be! However, it is still not a good idea. "Too hot" plus "fan" still equals "bad." Just arrange for your brooder to maintain a comfortable temperature for them to begin with.