How often do you clean out the coop so it stays dry?
With a child as young as yours I suppose you might not do it daily?
Drakes can kill your chickens trying to mate them (drakes have a long penis formed like a corkscrewer that will harm chicken and tear their innards out). They even...
What kind of ventilation does your coop have?
And how many ducks (females only?) do you house in there?
As already mentioned, chickens don't do well being in the same coop as ducks, they can't stand the amount of humidity brought in by the ducks.