That sentence sent chills down my spine.
Life is an end in itself. It is a not a means to the ends of others. Your children are no more my responsibility than they are my tool to serve me in old age. My pension and care in old age is not their responsibility any more than their behaviour is my obligation to put up with.
If this sounds strange and overly "individualistic" then let me frame the issue in a slightly different way.
If we accept that children are an "absolutely necessary part of society" because "they will work for our pensions and care for us when we are old" then what happens if people choose not to have children? Are childless (by choice) people parasites in such a society? If more and more people decide to not have children is there a point where this becomes a social crises to the point where we start forcing people to have children?
And what about children who decide to emigrate as adults? Is there a point where you prevent people from travelling in order to guarantee that you have enough people around to pay pensions and take care of old people?
Not that it should matter, but I write this a middle-aged parent of two adult daughters who I do not hold to any sort of moral obligation to take care of me when I'm old. If they choose to then cool, I guess, but it's their life to live as they choose. The mere fact that they exist does not automatically obligate them to anyone else for any purpose.