A 250W light even spaced away is IMO way too much for such a small brooder, even if the air temp in the brooder was 98-99° the IR radiation from that big of a light could overheat (give them a sun burn) the chicks if they can't get out of it directly shining on them...
Well to be blunt you are going to have to toss us a few bones of info before anyone can give legit speculation of cause...
Pictures, verification of temps, food and/or water availability, location of the dead chicks in the brooder, heat source and anything else relevant even if you don't think...