You'd need sweeping childcare legislation. A big part as to why the German TRF is fairly low is because West Germany never really jumped on the working mother bandwagon, and schools let out at 1 PM. So work or family, not and. East Germany had better childcare, and it was dismantled through the 90s. It actually was a large part of Ostalgie. Maybe giving the new Länder more autonomy to begin with? Or perhaps a more federal Germany?
Besides, why would a developed country, and a fairly dense one at that, have more than three kids to a family? What's the economic incentive?