What breed is your duckling? Have a video?
There are some breeds, specifically heavyweight breeds like Pekins, that will breathe a little heavier when laying down, even if they have a deep water source provided. If there is no labored breathing, nasal, or ocular discharge, I wouldn't particularly be worried.
In the case that they were showing some of the symptoms mentioned above, it would be better, for wider coverage, to use something like Doxycycline, or Enrofloxacin, as Cephalexin is more for skin infections like Bumblefoot. Please do understand starting an antibiotic when you don't know the underlying cause, can make the problem worse. For example, if you give a bird an antibiotic when the problem is fungal related, that will allow the fungus to better cultivate inside the lungs so it doesn't have to fight against the natural/good bacteria.
For future reference, if you ever do need to use the Cephalexin, take one pill, crush it finely, dissolve it in with 10ml of water. Mix thoroughly. That solution will contain 25mg of cephalexin per ml of the liquid. Using the dose in Plumbs veterinary guide ( 35-50mg/kg PO QID), you would draw anywhere from 0.64, to 0.91ml, and dose per pound, four times daily.