My old criminal law professor, Alan Dershowitz, used to say that all you need for premeditation is for the person to think "I'm going to kill this guy" a split second before they pull the trigger. In this case, unless it's involuntary manslaughter, I would think it's premeditated or nothing. If the guy intentionally left the kid in the car, he did it to kill him.
I agree. It's really black or white in this case. I see no room for any other charge, other than, possibly, negligent manslaughter, which I presume they have there. Again, motive is not a necessary element at all. People kill with no motive whatsoever, randomly. And, with serial killers, there is never a motive other than that they get off on it and feel compelled to keep killing. Maybe you're watching too many crime/lawyer shows.
However, as I tried to explain above, motive can become important in a case like this, just not because it's a necessary element of the crime. If the prosecution doesn't turn one up, then the defense is going to use lack of a motive as a defense - and there'll be a parade of character witnesses. OTOH, if the prosecution turns up that he took out a million dollar life policy on the kid last week or that there's a girlfriend waiting in the wings for him to become single and childless, then motive may again be an important element in the case. It's just that people kill for mindless reasons all the time - and are convicted for it...