I can't believe this thread is still going!! And is reasonably polite.
Let me add my two cents. I sat and watched a woman and her child (about 6ish if I remember correctly) walking around the gate area, years ago. She and her son would sit every so often, but would get up and wander quite often. We boarded. I was in the late A group. Well.....at the very end of boarding, this woman and her son boarded the plane....not surprisingly, there were no seats together. The FA asked for some people to change seats so the mother and son could sit together. No one offered. The FA then announced that the plane was not pulling away from the gate until the mother was seated with her child. A few passengers finally offered to move. At this point, the mother asked if there were seats closer to the front!!!
So, I understand that possibly her son was better off waiting until the last minute to board....that used to be SOP. But with SW, that won't get you seated together!! If being seated with your group is important to you, there are options. You can pay for EBCI and get a better boarding position. Or, you can book with an airline that allows you to choose your seat when booking.
It's up to parents (or others) to take responsibility. It isn't fair to expect others to change their seats in order to accommodate you. They may very well have their own reasons for choosing those seats.
I have offered to move....on many occasions. But I will not be badgered into it. It's just not my responsibility to make it possible for others to sit together.