I have bad knees and whenever possible, I get an aisle seat so I can stand up as often as possible. If I don't, I am in a lot of pain and have a lot of stiffness at the end of any flight of more than 1 hour. So, it would have a been a "so sad, too bad" response from me to that mother.
And, there's nothing wrong with that. I do not get why people with children believe their "wants" trump the "wants" of every other human on the plane.
And, for the record, I have kids. Have flown a BUNCH of times with them (many, many times) and have never, ever had an issue with getting assigned seats of exactly my choosing. I don't fly SW and this is one big reason.