Giardia is more common in water fowl. Generally you treat Giardiasis with fenbendazole or metronidazole. Fenbendazole is panacure granules.
I would have the chicken poo tested before I'd spend money to treat it.
Have your dogs retested after treatment as it often will take more than one course of treatment to eliminate the parasite. Giardia is associated with water "pristine mountain streams" - boil the water when you go camping. Don't drink the water in Mexico - Montezuma's revenge! So be sure these dogs are not continually reinfecting themselves.