I fail to see the difference to you whether the kid has their own seat or not, the only person who benefits from the child having their own seat is the airline. As long as the parents are doing their best to quiet the child that is all you can ask. I assure you they are almost certainly more frustrated with the situation that you are (assuming they are good parents, which it sounds like they were). Also, what do you want them to do, walk around the cabin and say sorry every time the child has a fit, of course they are sorry... it is not like they woke up that day and were like "Boy I really hope my child is a pain in the ... for all the people on the flight today, thats the whole reason we are making this trip." Now if the parents aren't making an effort to care for their child, I would totally see your point, but that does not seem to be the case.

Look I get it, it's of course better for their not to be a screaming child on the flight, but there is a pretty simple solution of getting noise cancelling headphones to solve this issue.