Let's sum up this thread and move on --- post partum depression a) does not remove free will, b) does not remove God's help/grace, c) is not an excuse to sin.
That's the bottom line ... that for a psychological condition to exculpate, it would have to be so extreme that the individual no longer knows right from wrong. There was a thread here about how drunkennes becomes mortal sin when someone has effectively disabled the higher faculties. That's the same threshold for whether or not a condition is of sufficient severity to render someone's actions something other than "human acts" (as Aristotle and the scholastics referred to them). So, when she says that a voice told her to KILL the CHILDREN, that's an admission of guilt, that she was aware that she was KILLING CHILDREN. She did not have a psychotic break where she thought they were gigantic spiders who had burrowed into her children and started to inhabit them while wearing their skin for cover, or hallucinated that she was merely tucking them into bed rather than strangling them. Besides, the evidence is clear that 1) she had planned it, and 2) she showed very little remorse after the fact.
She's guilty as charged and should receive the death penalty ... as anything else would be to declare her children so much garbage that can be discarded with impunity. Let's say someone butchered a dozen children, and the courts gave him 30 days in prison. That's a disgrace and an insult to the victims and their families. On top of everything else, if she gets off that sets a precendent that any crazy bitch can just go kill her children and claim she had some kind of temporary psychosis and depression.