Most criminals and junkies are bad parents and knowingly or unwittingly teach their traits through learned behaviour. Having children is a privilege. If they have committed crimes against society then this privilege should be withdrawn and any existing children removed and brought up properly in decent loving homes. Ethnic cleansing is a completely different thing which I totally abhor.
Given that most criminals will spend most of their lifes in prison anyway they are hardly role models or qualify as good parents. This idea is not so much to punish as to protect these vulnerable children.