There's definitely an argument for reintroducing capital punishment.
That said, an 18 year old single mother living by herself, and already in council care, she wasn't singularly responsible. This is not an excuse or sympathy for what she did... punishment should serve as a deterrent to others, although what she did is universally deemed unacceptable.
However, the father and local authority are as guilty as she is, and whilst she probably couldn't even name the father, the local authority and its staff should be made to carry some of the responsibility.
I suspect that this is why she received 9 years. Some digging may find that the LA haven't got away scott free.