First a note: The third entrance you found has a minimum sentence of 10 years in jail for child Manslaughter and no minimum sentence for adult Manslaughter, hence is nor the “same sentence” - per that law even if the Juri finds reasons for a sentence lower than 10 years in prison, the sentence cannot be less than 10 years in prison if the victim was a child.
The paper in the second link covers sentencing guidelines (which is formal guidance for prosecutors but not actual law, so they can disregard it) and how jurors actually decided (i.e. derived from de facto results), both of which as far as I can tell are much more common way sthan “by way of formal law” in shaping the sentences for killing of children are different than others.
Quite independently of all that, even just that first link (first result in the Google search) disproves that notion of yours that Murder is Murder.