The PP said that she "had no idea [that the Air Tran customers effectively have priority over the seating]!"
I've read AirTran's Contract of Carriage. I was unable to find anything explaining that AirTran tickets purchased through AirTran have seating priority over AirTran tickets purchased through SW. Perhaps you could quote it for me.
Then I went to AirTran's website to read its "General Information" under the heading "policies," which I assume is what you refer to as "basic policies." It states, "Advance seat assignments on AirTran operated segments are available for purchase at any time with Discount Coach and Sale fares." SW is mentioned several times on that page, but nowhere does it explain that this "basic policy" does not apply to AirTran tickets purchased through SW.
So, a person reads AirTran's Contract of Carriage and "basic policies" before booking tickets, and there is still a problem. This further illustrates the PP's point that the SW website does not clearly explain that AirTran customers who purchased through AirTran effectively have priority over the seating. And, the AirTran website doesn't clearly explain it either. Yes, the FAQs state, "Advanced seat assignments are not available on shared itineraries purchased via Southwest channels. Customers who purchase an AirTran segment on Southwest channels will be assigned a seat at the time of check in." But, this information isn't included in the "basic policies." Given that the basic policies clearly explain that advance seat assignment are available for purchase "at any time" and also reference SW, but do not address the seating priority issue, there's seemingly no need to read FAQs, hunting for inconsistencies.
The point is that SW could do a better job of explaining the seating issues with AirTran flights booked through SW. Yes, it might be contained in either SW's or AirTran's policies (basic or non-basic), procedures, methods of operations, and/or obligations. But, IMHO, a customer shouldn't have to read all of that (assuming it were publicly available) to understand what actions parents might take to ensure that they are seated next to their child.