How anyone could leave a pet, to say nothing about a child, is just beyond me.
Since it can never be proven as premed murder, negligent homicide will probably be the charge, however second degree murder could fit.
You're right to be concerned, as dogs will generally die before humans when confined in a hot car. Humans sweat: we're "water-cooled." Dogs pant: they're "air-cooled." Depriving a dog of adequate ventilation at high temperatures will fry the critter's brain.
Of course, the same thing can happen to people eventually (especially young children,) hence the tragic story that started this thread.