What type of climate are you in? It is not always necessary to worm them unless they are having symptoms, or if you are seeing an occasional worm. Most worms are too small to see without a microscope. Your vet could run a fecal float to check for worms.
SafeGuard is very safe for use in chickens. It can be used in chicks as young as 6 weeks old. Do you know how much they weigh? Dosage of the 10% fenbendazole horse paste is 0.25 ml (1/4 ml) per pound of weight given orally. For roundworms and cecal worms, give it once orally, and then repeat in 10 days. For roundworms, gapeworms, and capillary worms, give it for 5 consecutive days.