Congrats on the babies!
As for the sickness, I have no experience with this, so I don't know what you should do. Is the mom and chicks partitioned off from the rest of the flock? Or is she using the same feeder/waterer. I'd probably go on the safe side and treat everyone, chicks and all. My feed store dips all the chicks beaks in antibiotic, so it can't really hurt, right?