I'm thinking Aspergillosis, especially after you mentioning the bedding your using.
The straw can have fungal spores all throughout it. Depending on when and where the straw was harvested/baled, if it was in a wet area or conditions, could be moldy, and have fungal spores adhering to it and now that it has dried out, has become 'frieable' and easily released in the dust.
I would replace the straw with hardwood or pine chips, immediately. There is no treatment only sanitation procedures to eliminated the fungus.
You mentioned them eating and drinking normally, you didn't mention anything about their feces but if it was Coccidiosis, I'm sure you would have noticed their droppings and stated the condition of them....if it were cocci, you would notice yellow runny poop and in advanced stages bloody poop, at 1 week old I doubt seriously it is Coccidiosis....can you perform a necropsy on the dead ones? or have you already disposed of them?
Their kinda small but you might be able to determine the cause of death.
ETA- for auto correct