Japan is now a threat to the loss of human population, the reason as you say, if we think in a common sense, children are not a burden on the economy, precisely without realizing those who do not have children that is the economic burden, life is sometimes strange, that's the situation.
Its made a burden by the state. Worldwide the cost of raising a kid is way higher that 40 years ago.
40 Years ago a family with one earner had 3 kids. Now both are working and hardly have one.