Get marbles or clean pebbles into that open waterer today. Bantam chicks are small enough to fall in, get soaked and chill, and small enough to drown, and an open tray is the usual way it happens.
While you are in there, dip each chick's beak in the water once. Some of them do not work out drinking quickly, and for very small bantams a lot of keepers grind the crumble finer so it is easier to get at.