“What is the psychological impact of not taking children seriously?”
The premise of that question is slightly false.
Children are people, and in general, the reason I take people seriously, for example behaving politely to other people in the gym, is not that I’ve done the calculation that not taking them seriously would have a psychological impact on them such that in five years’ time they would be drug addicts or something, it is because they and I are humans, who are sharing the problem situation of being in the gym, and that leads to certain moral conclusions.
Taking Children Seriously is not the conclusion of a science that has determined that being mean to a child will later cause something or other bad, any more than it is in the case of adults. It’s a condition of rationality.
Deciding issues of how to treat children (or anyone else!) according to the effect you think it will have on them later is a mistake. It is the effect on them now that is the important thing. Later is incalculable anyway. It is not predictable.
We don’t decide moral issues in general by looking at the effects. Notice the double standard.
“Corporal punishment is known to have a long-term harmful psychological impact, so confiscate her iPhone or ground her for a week instead” might sound all right to today’s parenting experts if we’re talking about a child, but what if we are talking about a wife instead?
If the problem includes the value that you place on something that will happen five years hence, then that consideration can enter morally, but usually, we do not know that, because we cannot even guess that. Either of us might get run over by a bus tomorrow, and it will still not have been right to behave immorally today.
“Why is the effect now important?”
For the same reason morality in general is important.
Because there’s a sophisticated theory of right and wrong, and there are general theories of this, and the theory of right and wrong with people you don’t know is generally covered by the theories of liberalism, which are quite sophisticated, but they don’t go as far as relationships, but the basic logic of it is the same.
It’s all about not suppressing the mechanisms of criticism and error correction and creativity and so on. And those things create morality.
It is a mistake to suppress the mechanisms of criticism and error correction and creativity because doing so impedes the growth of knowledge.
Sarah Fitz-Claridge, Taking Children Seriously FAQ: ‘What is the psychological impact of not taking children seriously?’, https://www.takingchildrenseriously.com/what-is-the-psychological-impact-of-not-taking-children-seriously/