Society needs children to survive - someone has to pay the pensions after all, at least in common "societal contract" systems, otherwise people will have to literally work until they die.

For that in turn, society needs at least a replacement fertility rate, and usually there is a close negative correlation between wealth and amount of children (i.e. the wealthier people get, the less children they have), so it makes sense to strengthen the ability particularly of the lower classes to have children.