Im not sure she did (personally i reckon he killed her) but thats the type of thing the defence will probably claim and unless theres DNA evidence tying him to the killing itself (as a pose to disposing of the body) it becomes a somewhat he said-she said and KM is not likely to be viewed as a hugely credible witness by most juries (given her history) and even more so if any tox screens (i imagine the hospital where she is currently would have taken one) show that she had either taken illicit drugs or was off her meds at the time of the killing, you would imagine his defence attorney uses it to attack the credibility of any information she might give to police. They can try and create reasonable doubt if they can show the jury its not certain which one of JS or KM actually killed the poor girl.
Yeah, that's my concern, and instead of a murder charge, and conviction, it will end up with manslaughter charge against one or both. There may not have been intent to murder at the point it occurred, but actions afterwards point to covering up the death of a 9yo. Despicable actions, and on the face of it, two despicable people.