Ok, first get electrolytes and vitamins in the water.
Also, get poly vi sol (NO IRON)
Hard boil some egg and feed that to them.
Ducks need lots of water.
No medicated chick starter for ducks.
Brooder should be at 96 and98 degrees.
You can decrease the temp by 5 degrees every week (unless they show signs of being cold)
You might put an old tube sock in the brooder with them to cuddle with.
Sometimes, if you get chicks you think are dead....you can warm them up and hydrate them, and they will come back. Been there and done that several times!
My husband was about to throw away a chick he thought was dead and I felt bad for it and held it and walked around with it in my hand for a few minutes waiting........well a leg kick was all it took before the triage unit opened in our house! The chick was warmed and drop by drop fed sugar water to perk it back up and hydrate it. Then electrolytes and vitamins. Then drop by drop Exact wet baby bird food. She was back up and running completely normal in about 8 hours of being very vigilant.
Good Luck with your chickies! And I am sorry you had to loose some babies......