I agree with everyone else. Your brooder is not ideal. A box with solid sides will not have a cool side (a crate or cage is better with a heat lamp) but a heating pad or plate is a much better option. It looks like you have the food and water under the heat lamp, which is the wrong way around. The food and water particularly, should be in the coolest part of the brooder. Did you check the chick that was cheeping for pasty butt? If we are right about them overheating, pasty butt is much more likely due to dehydration. Once they get too weak, their necks will go limp.
Can you tell us what you are feeding them? Chick crumbs are normally small enough for chicks to eat without further processing. What is it that you are grinding up?