A little coconut oil warmed to room temperature (it would be liquid then), dabbed on the end of the bill to avoid risk of aspiration might be worth a try.
Per Storey's Guide I sprinkled a tiny bit of chick grit on the starter crumbles from early on so that they could chew non-boughten food after a week or so.
Do they have water with food 24/7?
General rule of thumb - brooder temperature starts at about 32C, then drops 2 - 3 degrees C each week for ducklings.
It could be an infection . . . might the food have a little mold in it?