I took a look at your brooder. Is that sawdust I see in there from those pellets that break up when you add water (horse stall bedding pellets)? Do you have any chick grit in there for them? Because if not, it could be they got some sawdust or wood chips stuck in their crops and couldn't get it out. They may have eaten it instead of or in addition to food, and that was the end of it. Takes them a few days to understand what is food and what is not.
You've gotten great advice about the heat plate, heat lamps, and brooder temperatures. I suggest you add a small container of chick grit.
For my day old chicks, I mix the chick grit in with their food to be sure they get some, and at about a week or two I feed it separately. For the Quart chick feeder, I fill the feeder 1/4 way, add a handful or so of grit and mix it up, add 1/4 more food, handful of grit and mix, etc. Just so the grit is scattered through and you know they get some, but they don't just fill up on grit. Once they're old enough to understand the difference, they can eat around the grit if they want to, or they can pick it out.