Do you see any worms in the poop, and if so pictures are helpful. Many worms are microscopic, but a fecal float by your vet will tell you the types and if worm larvae are present, along with looking for coccidiosis. Valbazen and Safeguard liquid goat wormer are both very safe wormers commonly used in chickens. Those treat roundworms, cecal, capillary, and gapeworms, but are not the best for tapeworms.Both require a 14 day egg withdrawal time, although some people do not bother. Levamisole or Prohibit powder to use in the water can also be used with the same egg withdrawal time. I would get the fecal float if possible to make sure that they have worms.