Alright first you say your new ducklings are only about two weeks old alright I can work with that ..
First how warm is it where you are ?
Next the momma duck always keeps its baby's warm and all it takes is a regular light bulb not a heat lamp as most people think .
What are you feeding your ducklings is very important and a hard boiled egg will work a duck egg even better but an egg has everything a baby needs to grow and be healthy, I learned this little trick from my grandpa in the 1960 when I was but a lad and he learned it when he was a youngster so if it has worked all these years there must be something to it, ya kind of like putting honey on a open wound it will help seal and heal the wound well the Romans used it and the Egyptians and it will work today ..
Oh and try some ACV in the drinking water it never hearts .
Now I only hatch and raise several hundred chicks a year so I might know what I am doing