Unfortunately that answer just leads to punishing all due to some people.
Even if we could change the benifit system to value the position of the parents whe they choose to have children.
You would still be punishing the children for the actions of an adult.
And ignoring the fact that your policy basically forces abortion on anyone who accidentally gets pregnant.
I hate to bring of the pro life pro choice debate. But when you start pro forced abortion I think pretty much everyside is going to respond negatively.
Simple fact is. Birth control is not perfect. And abstinence even less so. And using finances to force anyone in poverty to abortion when they accidentally get pregnant. Or to force all poor married couples into abstinence. Is morally way worse then the idea of supporting the children.
And unfortunately. Anytime you try to value welfare on the right or wrong of having children. Those are the choices you are making.