An observation is because women's unpaid labor is valued at zero people then assume the cost of raising children to adulthood is also zero.

Classic neoliberalism is to add 'without spending money' to questions like how can we reverse falling birthrates.

I don't think anyone assumes that the cost of raising children is zero... quite the opposite.

I don't think its key, either. Can say about Japan specifically, but in the West I think it's down to people wanting to have a life, things, holidays instead of having children. It is a choice that is about the whole commitment and sacrifice of having children no matter what.