in my experience it's not that they are not good mothers, but both Wood duck and Mandarin duck hens are pretty nervous and agitated when they first come off the nest and if there are other ducks in the pen with them, the other ducks might also start chasing and pecking the ducklings as they jump out of the nest box. So the hen are trying to call all the babies out, while protecting the ones that are already on the ground from the other birds in the aviary. The hens might be swimming back and forth, in and out of the pond calling the babies still in the box and the ones that already jumped, are trying to keep up with mom, following her in and out of the pond, at times they get confused and start following other ducks which turn around and attack them. it is a mess ! So the babies get exhausted and too wet from going in and out from being bullied around by other adult ducks and they end up drowning.
If you cannot be there tomorrow to watch them when they come off the nest, I would actually cover the pond if it's possible. Don't drain it cause they will fall in it and never get back out. But cover it with a board or something (I don't know how big it is).