My mother is afraid of flying and gets extremely nervous. Once a flight is booked, I get constant updates on weather (even if it is months away), how afraid she is, and she starts discussing how she doesn't even want to go on vacation. Once we get to our destination, she starts worrying about the flight home. I pay extra to make sure my mom sits next to me on flights because I can help keep her calm during the flight. If a mother and a child is on my plane and they did not pay extra and now can not sit together, I will not move for them. I planned ahead to make sure my mom was taken care of, I expect parents to do the same. Also, I will not document my mom's anxiety with the flight attendants. She does not need special treatment, just someone she knows sitting next to her.
As a side, when I was a child we booked a last minute flight because my grandmother had died. We could not get seats together. My mom and some of my siblings sat in the front of the plane. I was in the back of the plane on my own and my dad was somewhere in the middle. Guess what, I survived the plane ride and we did not need to ask anyone to move for us. In fact, I remember the lady who sat next to me was extremely nice. So, just cause a family has a child doesn't mean there is a need to sit next to each other.
Your arguments seem to imply parents don't need to have any responsibility for making sure they do what is best for their kids. The only time I think it is the airlines responsibility is when a parent books seats with their children sitting by them (even if they have to pay an additional fee) and the airline moves the family and separates them.
And before you can reply, I get it, you are going to call me selfish. I guess what you call selfish, I call being responsible and planning ahead.