We recently lost our Airedale, Piper, to cancer. We would like to get another dog soon. We are open to adopting a shelter dog, or getting a purebreed. We have 3 sons, ages, 6, 4, and 8 mos. We have lots of experience training dogs and would like a breed that is of medium size under 50 lbs, smart, friendly yet protective, and not of intense energy. We give our dogs a daily walk and have a partially fenced but small yard. What breed should we look into?