This one is wild. You want to sit next to somebody's crying 2 year old? Go nuts. Change their diaper while you're at it.
What I'm saying is, if you do it this way, you're now leaving the decision up to the parents. And some parents will choose not to pay. When that happens – because it will happen – I don't want to hear people complaining about having to sit next to other people's kids. Everyone was treated equally, a choice was given, a choice was made.
The other option is, we say as a society that here is a situation nobody wants, we all see that, so we're all going to collectively agree to set things up in the parents' favor a little bit, thus doing something nice, creating an outcome that is better for everyone, but at the cost that some parent seating gets subsidised by others on the plane.
Just laying out the options. Classic individualist thinking will say, I don't want the government to decide for me that I should subsidize. And thus some people will end up sitting next to somebody's crying 2 year old.