With babies and young children, you want to use a fast lens (f1.4 or f1.8) to avoid using flash. A flash would distract them and your shots would appear un-natural. I have young children myself and I get excellent shots with the FA31mm f1.8 (AF), VL 58mm f1.4 (MF) and CZ 85mm f1.4 (MF). I get my best pictures without flash
The FA31mm f1.8 is a fantastic lens and a fast lens. Go for it.
For portrait, a focal length between 30 to 100mm is typically recommend. With a 30mm, you will need to come closer while, with a 100mm, you can stand back.
I would not use a macro lens for a baby. Since you already have the FA31mm, use it.
We look forward to see your shots.