OK assuming that they did it - they could be charged with either.
Murder is considered to be malum in se, that is an act which is evil within itself. Unlawful killings without malice or intent are considered manslaughter.
But generally murder must be with the intent to kill or with the intent to cause harm whether to that peson or to another (ie if you aim and shoot a gun at someone but kill someone else - that is still murder)
So what is it? There was intent, the deaths were a result of a deliberate criminal act and the crime was one that would reasonably cause harm to person or persons (as opposed to shoplifting etc).
Isn't there such a thing in the states as Gross Negligent Homicide? These parents had a duty of care to those children and , unless mad, must have known that there was a danger of harm.
However - they may still be innocent.
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