One Photographer Only Please
Have you ever taken your child to her doctor and pulled out your own stethoscope to use while the doctor was examining your child? How about when you took your car in to be serviced? Did you ever bring your own tools and work along side of the mechanic?
As a photographer, I expect the same courtesy shown to me as you would to a doctor or any other professional doing you a service.
I expect to be the only one holding a camera during a session.
It is a courtesy not only to myself as the photographer, but also to the subject(s) being photographed. It is distracting to the person/people being photographed and detracts from the ability of the subjects to engage with me properly. This goes for sessions done in the studio as well as on location.
Do not bring your own camera or use your Smartphone camera during a session.
One photographer per session only please!
Thank you for your cooperation.
CM Photography Blog (Winnipeg Newborn, Child, Family, Baby Photography) 
Great point. In fact it can ruin the photos that clients are paying for. Unique moments come and go and are lost forever. When you hire a professional photographer like Chris, it is because you want to freeze these moments in time. Many cameras have automatic flash, and or red eye reduction lights that will totally ruin a shot if they are activated in the middle of a session. Play it safe, leave them in your pocket.
Good point. Those unique moments are over in a flash and if the child is looking in the wrong direction it can ruin the shot. Thank you for your comment.
Chris