well theres nothing demonic about it. There are god of death, but theres a difference between that and demons. No where in it does it say they are evil.
and I think it depends on the kid. If they are smart and can figure out the situation, and respectfully understand it, why not? True, its not for little kiddies, but age 12 and above is fine. I like Death Note because it had everyone questioning their morals, and think it can be a decent test for the average person's depiction of true right and wrong.