Feeding them chicken feed, in general, is not bad. Ducklings can be raised on chick starter. Feeding them layer is another situation all together. I have never done it, but if you read the Holderread duck book, they tell of a story of losing a large number of ducklings when their feed dealer accidentally added too much excess calcium. They recommend highly against feeding ducklings excess calcium, so I personally would avoid layer (for ducklings, adult ducks do fine on it) unless you are only using it for like a day.
As far as what to feed the bantam ducks, I would highly recommend limiting food intake by the time they are 2-3 weeks old and lowering the protein. Body size in bantam ducks can be greatly influenced by feed intake. By the time mine are 2 weeks old, I have them on about 75% Flock Raiser mixed with 25% oats and also allow them to forage or give them fresh greens. If you don't do something similar, their size will increase (especially over multiple generations). Numerous authors and duck raisers have made note of this. I have tested this with Mallards and I can double the size of a Mallard line in only two generations. If your goal is to keep the small size, this is good to know.
