Do family physicians treat infants too?

Family docs. May be board certified and trained, and know how all of the body systems and organs work. They almost always know their limitations and when to ask for consults from specialists. They also know when to send to a peditrician.
Most do. Their training includes them.Some have an age range below which they leave to others. The whole idea around family practice is to monitor the whole family and be aware of the interactive dynamic/influence of each on one another. Knowing what you know and when to refer out is one of the skillsets that help them do their work.