Not even a child's skin looks that smooth in an unretouched photo of that vintage. The tonal values are higher than what the film would produce naturally under that lighting, so I'm fairly sure they've been pushed up a bit with some graphite, and as mentioned, there's probably a little sharpening going on with pencil and etching techniques. Check out the fingers on the baby's hands--no way to keep those still. They were a little more careless with the fingers on the older child's right hand, since it's more in shadow. That heart-shaped cowlick on the baby's head--probably defined with some etching.
It was just what made a portrait "professional" at the time.