I would second what bobbi-j said above. I have two dogs, a 10-year-old mix (maybe some Australian Cattle Dog and some Norwegian Elkhund) and 2 1/2 year old Anatolian Shepherd Dog. The ASD is an ancient livestock guardian breed, so his instincts should be to guard and protect the chickens, but when he was a puppy he required constant supervision around the chickens because the urge to chase was pretty strong too, expecially if the chickens were flapping around in a dirt bath. Up untill this summer, I always had the dogs teathered if the chickens were free ranging because if something startled or excited the chickens and they started running or flapping, the dog's prey drive seemed to kick in. At 2 1/2 I have good reason to feel confident that my ASD understands that the chickens are not to be harmed and can leave him out on chicken guarding duty unsupervised.
So, if I were you, I'd introduce your two family dogs to the chickens one at a time where your focus was on one dog and watching their body language to see if they exibited any prey behavior: hard focused stare, tail up, lip licking and/or hunting crouch or really excited behavior, that might not be aggressive but could still result in injury to a chicken. Correct any unwanted behavior before it escalates and your dog should get the idea that they need to leave the chickens alone. Once I felt relatively confident that the dog had the right attitude, I'd probably find an outdoor activity like working in the garden where I could observe the dog while the chickens are around and see what happens. Teathers of some type are helpfull to, because even if the dog lunges at a flapping chicken, chances are the chicken can get out of reach.