I think it's important to separate out three things: the horror of the crime, the culpability of the mother and the culpability of others.
In terms of the horror of the crime, an innocent child has been murdered by his mother and main carer, the one person who he should have been able to trust. Nothing can diminish the horror of that and his disabilities don't come into it at all, except to make him more vulnerable.
In terms of culpability of the mother, the CPS has accepted the mother's plea of manslaughter by diminished responsibility. So it accepts that her mental illness made her significantly less culpable. However, she has still be found guilty of manslaughter and will be sentenced either to prison time or to a hospital order (depending on an impartial judge's assessment of her culpability). So she hasn't 'got away with it' and has been recognised to bear some (although reduced) culpability. That is fair.
In terms of judging the culpability of others, I hope justice is done here. The roles of both the father and the local authority should be scrutinised and they need to be held to account if it turns out that there has been neglect of this poor boy on their part which contributed to his death.