Thank you all for your input.
I have puzzled this out and I have come to the conclusion that the deaths were due to an inexperienced mommy, an inexperienced chicken owner and the night temp going into the upper 40s.
My rationale for this is as follows:
There were no signs of respiratory illness.
There were no signs of digestive illness.
There were no signs of parasites.
The only thing these chicks had in common was they were sluggish, weak, and
cold to the touch. The one chick that survived was the first-born and the largest. She was following her mother around, while the others were either alone in the run or two together in the coop.
I have bulked up the shavings and added straw. I will be installing a bulb for heat before I hatch a brood in late July again and leave them to their mother's care.
And, as a precaution, I won't let visiting kids toss tree leaves into the coop/run.