You might have kids, and then they work the farm, then you manage the farm and slowly the children take over the manual labor and hard work of it. In old age the investment in the children pays off and a reciprocal relationship is formed where you take care of the grandchildren and your own children take care of you.
Now that is flipped on its head. The parent makes the lions share of the investment in the child, but the benefits of the child is largely socialized. Want daycare, food, recreational, extra-cirricular activities -- basically anything other than public schooling you pay taxes for already? Go fuck yourself.
But once the children is grown up, well well well we are a society here! Tax the shit out of the kid, spread the social security benefits around to everyone including people that didn't raise any children. And if you directly want a piece of the investment from the children, as people got in the old days, well then go fuck yourself you greedy selfish bastard -- it is only morally right when all of society does the exact same thing to the kid.
There is every possible incentive in today's society to encourage others to have kids, ensuring your own retirement, but to reneg on doing it yourself because some other poor bastard can front most the costs and then you can tax the shit out of the kid for your retirement / social benefits. I think children were a rational decision in Darwin's day, now they are definitely not, because you are on the sucker end of a tragedy of the commons deal.