I expect she has been charged with 'gross negligence manslaughter'.
Gross Negligence Manslaughter
This is where the death is a result of a grossly negligent (though otherwise lawful) act or omission on the part of the defendant. The law in respect of this has been clarified in the case of R v Adomako (1994) 3 All ER 79 where a four stage test for gross negligence manslaughter known as the Adomako Test was outlined by the House of Lords:
The test involves the following stages:
a) the existence of a duty of care to the deceased;
b) a breach of that duty of care which;
c) causes (or significantly contributes) to the death of the victim; and
d) the breach should be characterised as gross negligence, and therefore a crime.
http://www.cps.gov.uk/legal/h_to_k/homicide_murder_and_manslaughter/#voluntary
Murder and manslaughter are both common law offences and both have a maximum life sentence.
In this case a charge of manslaughter is appropriate because it would be very difficult to prove that the mother deliberately starved her child with the intention of causing him to die.