No-one's claiming that it's unreasonable or unprecedented for kids to be noisy and disruptive due to (among other reasons) still-developing brains that can't fully process social norms and responsibilities.

We're saying that, given that everyone knows that fact, the parents who choose to bring poorly-behaved kids onto planes are being selfish and irresponsible.

The kids are mostly blameless in these situations - they're still developing, they can't (depending on age) be expected to be fully responsible. It's the parents that are selfish shitbags.