In 2004 Phillip Longman wrote an article about declining birthrates around the world with a lot of data and many ideas. I don't agree with all his speculations or proposals, but he did mention a really intriguing idea for solving the both the Social Security low fertility problems of the United States in one fell swoop:
Governments must also relieve parents from having to pay into social security systems. By raising and educating their children, parents have already contributed hugely (in the form of human capital) to these systems. The cost of their contribution, in both direct expenses and forgone wages, is often measured in the millions. Requiring parents also then to contribute to payroll taxes is not only unfair, but imprudent for societies that are already consuming more human capital than they produce.
This proposal is much more logical than a straight tax credit. The Social Security deduction should vary from year to year based on current population projections -- that way we won't have to fine-tune the incentive once it starts working.