She can't successfuly plead diminished responsibility, to reduce the offence to manslaughter; this was too premeditated. As to a devil telling her to do it, well, that's what defendants try on from time to time (the Yorkshire Ripper tried 'the voices defence').
The question is 'And did this devil tell you to burn the body or conceal the killing? Either you believed it right, in which case you wouldn't hide the killing and burn the body, or you thought it wrong. If wrong, how were you under duress to force you to do it, to kill your own child, the worst act you could do ?'
Clouting the child is the action of someone sane although a strict disciplinarian beyond normal bounds, but who happens to have gone too far and killed the child, without meaning to do so. That puts her into the murderer category, since she would have, on any reasonable view, have been intending to cause the child some really serious injury, though in temper at his failing to perform to her high standards .