Sorta related...I'm flying with DS7 on Southwest. Paid extra for early bird check in. Odds should be pretty good that we'll board toward the front of the line so getting seats together shouldn't be a problem, right? This is his first time flying and if we are separated, it'll be a big problem..
This is not worth worrying about. OK? Two people, paying for early bird? There is no problem. If you were the last two people to enter the plane, there might be a concern, but you won't be.
But, it is ALWAYS worth it to talk to our children about the "what ifs" of IF you are late, delayed, etc, and do end up getting on the plane late. My son has flown a lot, but on our last flight I realized that we didn't have seats assigned, then we were assigned 5 rows away 2 hours before the flight (not on SW, obviously). We'd started the morning off with chaos at the airport, with east coast weather messing everything up all the way out to the west coast, having one of our flights canceled and needing to be reassigned, etc etc etc. So when I figured out the seat assignment problem on our second flight of the day (we were traveling on miles and were put through the wringer, LOL), I spent LOTS of time talking to him about what to do and how to act and how totally OK he would be IF we couldn't get anyone to switch. (end of the story, I managed two switches with my seat, and we ended up sitting just one row apart, he made BFFs out of two grandmotherly seatmates, and wants to sit alone forever more!)
So talk talk talk with him about that sort of IF, just in case there's a flat tire on the way to the airport, etc.
But the reality of your situation is that you will almost certainly get seats together, especially since there are just the two of you! Plan plan plan, but don't *worry*!