I taught my kiddo to call someone by Ms/Mr.
Firstname or Lastname (whichever is known). He knows that if the adult says it's ok to call them by their first name then he can do that. Everyone with a title (dentist, ped, teacher, etc) starts with the title. We even know some people that are Capt Lastname and Sgt Lastname because I'm required to show that respect due to my job. I'm sure ...