So you agree that some people shouldn't have kids but your response is "it's still never going to happen". Why? Why isn't it going to happen? We have families where generation after generation are nothing but criminals. Cut it off at the bud. Do society a favour for a change. Why should we, as a society, have to put up with the persistent criminal element? Let's face it, we're already overpopulated and using excessive resource, anything to kerb this is a benefit for not only humanity but life on the planet as a whole.
China had a single child policy - perhaps it's time we implemented something similar - at least until people show that they can control one child? I've given the example on here before of a mother to several children at my partners school. She has had 7 children taken off her and in to care. She keeps popping them out and they keep getting taken off her. She's pregnant again. Give me one good reason why this should be allowed and why society wouldn't be better served by having her tubes tied. We're already paying to look after 7 of her kids who, thanks to her, have had an atrocious start in life.
As for your example, I've rarely encountered a situation where one out of several children is the only issue. However, my proposal would be in a situation where money is unavailable for reparations - time allowing - that said parent performs unpaid work for the community or they agree that their child is criminally responsible in which case the child faces criminal charges and the correct punishment.