Unless you're the very, very back of the line, it's unlikely that you'll be separated.
I exclusively fly Southwest, and often travel in a party of 3 (and often with just 2), and I have no problems sitting with my crew. I usually check in right at 24 hours ahead, and I usually end up in A boarding or even beginner B boarding. I might not always get to sit in the very front of the plane, but I can usually get 3 seats together. Even at B-40s, I was able to sit with my travel partner and because it wasn't a full flight, we had a private row. We sat towards the back, but we had extra room.
When you check in, all three of you will get checked in and should be in order "i.e. A 25, 26, 27" so you should be going together in line anyways. It's a matter of finding an empty row. As long as you're not C boarding, you should find an empty row.
They do the "Business Select" boarding as well which reserves A1-15 positions for people willing to shell out stupid amounts of money, but I have not seen more than 2 people ever take advantage of it. So there's 15 spots everyone moves up in line. (Unless someone wants to pay to upgrade to Business Select, which they do ask before they start boarding...)
As gwynne stated, Family Boarding is an option if your kiddo is six or younger.
You can upgrade to Early Bird Check-In and it will automatically check you in, but it doesn't really guarantee you an "A" boarding position, either. They note that in the fine print. You could very well get a lower boarding position anyway, but you'll be $15-25 lighter in your wallet.
I would also absolutely use a computer or the Southwest Website and NOT their app to check in. Their app is garbage when it comes to checking in for flights. I have much better luck using a computer or their actual webpage on my phone than their app.
