Presumably he already has told them his number and preferences. Defaults are fine, but you don't want your preference to get reset to default every time, and assuming that only the mother of a child should be contacted in all cases is a terrible default. The person who made the appointment and who is bringing the child to the doctor should be the one contacted by default. There is no reason that the mother of a child should be considered the default guardian. That is an incredibly dangerous assumption to make in many circumstances.

Edit: This reply was written to a response that got completely rewritten in an edit. It may not make as much sense