I don't think child benefit should exist. Maybe there is an argument for a first child since we assume we have the one life and it seems harsh to tell someone they can not contribute to future generations simply because they are poor, but beyond that ? No, we have too many people here as it is.
I don't think this issue is easy to answer. It would be inhumane to let a child starve/suffer just because a parent has failed to provide; but when the State says, "don't worry, have as many as you like, we'll get the taxpayers to pay your bills for you", then this is not a good message to send. How do you draw the line between not being used as a sucker, and keeping the dignity of someone down on their luck, intact ?
Whatever you choose, it's wrong, one way or another.