It depends on a lot of thing. Since you have small kid you would be able to boarding in B group after A group which maximum 60 people in front of you. You may be get lucky getting A group and the chance sitting together very high if you check in exactly 24 hours before the flight.
Based on my experience if your flight is very early in the morning, you have the chance getting A group. If your flight is late on the day, the chance is very slim since someone has the early flight and connected with your flight they would check in for both flight at earlier than yours.
I found most of the time you would be able to sit together since you depart before B group unless a lot of older people in the flight or the plane has a lot people connect through your departure so they already get good seats.
In general, people who fly SW are very nice so your chance is very high.
