They are definitely getting too hot. If you are using a 250w Infrared Heat Lamp bulb, good choice, I use them also sometimes as they are best for freshly hatched chicks up until 3wks old. It helps keep the vent (butt) pecking down to a minimal. That being said these bulbs needs to be a minimum of 18-20inches away from the top of bedding, at this height its around 110-115°F directly under the light on bedding and only heating one side (half) of the brooder floor. The other side should be shaded or somehow blocked from the light/heat. Looking at your picture, you have zero area for shade allowing the chicks to escape from the heath. Anything closer to floor, example 14-16inches the temperature jumps to 130-140+°F which is a death sentence. Easiest thing to do is buy a couple of small cheap digital thermometers and place one on the brooder floor directly under your light, and one about 6-8 inches out from center of light focus, leave them for 10 minutes and check it. Adjust distance from bedding accordingly. And don't beat yourself up! I learned from experience and had the same thing happen. About 6 of my very first hatching died because of this. I didnt know the light got that hot and when I checked it using method I just explained... it was about 150°F and please know, I left them briefly to go feed, water, and collect eggs from the rest of my flock. When I come back to the brooder they were already gone, so this only took about 15minutes from the time I put them into the brooder.
Also, I would like to add... watch and monitor your chicks for a while. If you see them with their necks kinda stretched, their heads into the air, beak up, with their throats throbbing (kinda looks like as if you would imagine a chick gargling water etc) This is a sure sign of overheating and stress and that something isnt right.