Okay, here, I looked up the specifics. In Illinois, where I live, negligent homicide is a Class 2 Felony, with a maximum of 14 or 28 years, depending on case details. Source.

1st Degree Murder is 60 years. Source.

I’m not a lawyer, but instead a layman. That said, I argue that based on the law as cited, the parents had no lawful justification, and that their actions carried a high likelihood of death. Obviously a lawyer making such an argument in court would by necessity have to back that up, especially if they trotted out religious nonsense as a “lawful justification,” but that’s what they get paid for.

A person who kills an individual without lawful justification commits first degree murder if, in performing the acts which cause the death: (2) he or she knows that such acts create a strong probability of death or great bodily harm to that individual or another;