“What is the psychological impact of not taking children seriously?”
Whatever might happen in the future, it will still not have been right to behave immorally today.
now
“Is the purpose of taking children seriously to avoid the harm coercion would do?”
Even if childhood coercion has virtually no effect, it would not change what it is right or wrong to do to people. And it is not right to do things to people that will impair the growth of knowledge.