I'd expect them to assert that paedophilia does harm. That's the obvious resolution.

Court are not supposed to investigate whether the child is emotionally harmed by the experience but whether he or she is under a certain age threshold. You could certainly imagine a legal system where psychologists are always asked whether a given child is harmed by having sex instead of a legal system that makes the decision through an age criteria.

I think a more reasonable argument for the age boundary isn't that every child gets harmed but that most get harmed and that having a law that forbids that behavior is preventing a lot of children from getting harmed.

I don't think you are a bad person to arguing that we should have a system that focuses on the amount of harm done instead of focusing on an arbitrary age boundary but that's not the system we have that's backed by societal consensus.

We also don't put anybody in prison for having sex with a 19-year old breaking her heart and watching as they commit suicide. We would judge a case like that as a tragedy but we wouldn't legally charge the responsible person with anything.

The concept of consent is pretty important for our present system. Even in cases where no harm is done we take a breach of consent seriously.

Actually I'm under the impression that the ‘standard’ resolution is not about the “harm” part but about the “want” part: it's assumed that people below a certain age can't want sex, to the point that said age is called the age of consent and sex with people younger than that is called a term which suggests it's considered a subset of sex with people who don't want it.

(I'm neither endorsing nor mocking this, just describing it.)