It's not the expense that is scary but the risk.
Most of what children need, healthcare, food, shelter, supervision. Is directly tied to employment of the adults.
If an adult loses employment you lose all pillars to support your child.
The social safety net in the US is not sufficient for people to feel safe in having children.
Further, the argument of why people should have children so the economy is supported is very shallow. I am not raising children so the economy can have more consumers.
As a society we should have larger goals then the next quarter results of publicly traded companies.