I think the argument goes that if you pay towards some of the costs of families with children the benefits come both.....
a) now, in that it means more parents can afford to go out to work and pay tax, and b) in the future, when those children are adults paying tax, which is then used to pay state pensions (for those with and those without children).

The key difference with state pensions against private pensions is that state pensions are not saved forward into a pot of money to be paid back, they are paid out of the tax revenues being paid at the time. With an aging population we will need more people paying out for all of our pensions in the future - and hence more children needed.

Or something like that.