I feel for you---sorry to hear of your loss. Raised chicks for five years now (not a bunch of them, but about 40 meaties the last three, and about 45-50 day old layers and bantams in years 1-3)...thought I had it down, until last month when I was so excited to get my first batch of layers in three years, ordered them from MI before they were even a glint in their daddy's eye...waited the 5 weeks for them to be laid, then hatched and shipped--22 arrived by mail all safe and sound, in the midst of the cold snaps we'd gotten. Had to revamp my initial plans for the brooder (I have an awesome one out in the coop ready to go) because it was so cold...so they were in my mudroom. They did great the first day and night...then I was totally shocked to find a lifeless body the third morning. One of my precious wheaten Marans babies hadn't made it--not sure if she was smothered by her buddies or if she had some sort of unseen defect as there were no outward signs. Been a little paranoid since but the remaining 21 have done just fine, and I did end up moving them outside to the lovely brooder that had been waiting for them after about 2 weeks, with additional heat to be safe.
Best of luck on the rest--sounds like you were doing everything right, within your power. Sometimes it is not preventable. Feed, fresh water, warmth, enough room to be able to move away from the warmth, good bedding (not cedar or newspaper), and a safe brooder...beyond that, it's up to them!