Simple solution: 1%/year tax on wealth and give these money to families with kids. The tricky part is wealth is often stored as shares and if you tax Bezos, he'd have to sell 1% of his shares, thus driving the price of those shares down; then if some hedge fund keeps buying the shares he sells, eventually he may lose control over his company, which is unfair. We might want to define wealth as savings above 5M dollars. This would be easy to work around: register a few companies with highly complex structure of ownership and spread money across them.
There's much more economic uncertainty for the average family as a result of two things.
1. Corporations innovating the labor market and successfully squeezing increasing amounts of wealth out of the workforce: successful union busting, ever more cleverly exploitative labor contracts, the gig economy, unpredictable scheduling, metrics and surveillance, etc.
2. Fewer funds proportionally available to labor thanks to increasing wealth capture by a tiny minority.
It's not only expensive, but it's statistically difficult to overcome the uncertainty and establish a homestead you can believe will prove stable over a kid-raising time frame.
It's easy to understand why people are hesitant to start families in such family-unfriendly economic conditions.