Our friend Julia moved to a small free-range pig and poultry farm when her first child, Jedd, was a preschooler. When her second baby was born, she would strap him on her back every morning so that she could go to the chicken coop to pick up eggs. Jedd would chase and ride the chickens—and sometimes taste their feed and touch the fresh eggs. A couple of times, she even caught him chewing on something he had picked up from the ground.

At first, all of this caused Julia to freak out. But once she realized that Jedd wasn’t getting...