I suggest that you do some requirement calculations first.
Lets assume that a baby cry has a lot of energy at 100Hz
Sound speed in air at 20 degrees C is a about 340m/s.
that gives a wavelength of about 3.4 meters, so your required distance is less than a wave length.
Assuming a baby is in a room , not much bigger, the baby cry probably has some standing wave characteristics. If this is the case there will be places within the room that are louder and quieter that are only loosely related to the distance between the baby and your microphone and if there is a relationship it will be weakly proportional to $1/r$.
I suggest you do some more experimentation along these lines. Make some of the assumptions more concrete, like the the frequency content of a baby cry. 100Hz is probably too low. 1000Hz changes a lot of the first order physics assumptions. Does moving your microphone have a $1/r$ relationship or are there loud and quiet spots in the room.
As some of the other answers suggested some amplification, how much amplification can be tolerated. An extra microphone can give some diversity gain but how far apart and how to process should be considered.
You really should move to knowing your problem better because those will lead to better solutions.