Strengthen immunity by increasing contact with natural environments

My mom is a family and children's photographer. She likes to photograph kids in their backyards or in local parks, sitting in the grass. She often encounters small children who are unnerved by the feeling of grass as they have never touched it before. Also, parents will ask if their child could sit on a blanket, because the ground is dirty. I'm really curious how this attitude started. It is so alien to me.