Many countries tried to solve low birth rates by offering rewards.
What must be understood is that these rewards themselves are virtually meaningless and low. The mere cost of raising children outweights these rewards.
It is more profitable to be without offspring.
So naturally, to counter, state must make it more profitable to have offspring, and not at all profitable not to have offspring.
So, a tax on people without children!
The less children you give birth to, the more tax you pay!
The tax should first apply on people who are 18-22 years old, because highest female fertility is at ages 15-22, and 18 is when you should be paying taxes.
Some countries have very low birth rates, such as Japan and South Korea, and unless they introduce some punishments for not having offspring, these countries will cease to exist in few hundred years.
The lowering of birth rates is a slippery slope. When birth rates lower, population gets older. Older population is less fertile, causing further decrease in birth rates, which further makes population older.
Its a cycle which, if not broken, guarantees country's collapse.