I think it's fucking barbaric that these two pieces of shit could be walking free
They may be released on licence, which means they will be supervised and may return to prison if they breach the terms of the licence. So a level of freedom, yes, but not completely free. Not saying this is how it should be, just clarifying the current system.
The Crown could appeal the sentence on the basis that it is unduly lenient. From the reports, I believe the mother and her boyfriend were convicted of unlawful act manslaughter. The sentence is pretty much at the top of the allowed range for a category B offence (8-16 years). To get a higher sentence, the prosecution would have to convince the courts that this was a category A offence. I may be wrong but, on the information available, I doubt the prosecution would succeed.
Note that the age of the victim does not affect the category of the offence. It is, however, one of the factors that pushed the sentence towards the top of the range for the category.