Barbie is also a fantasy, so I'm not sure how Harry Potter is different, genre wise.

SPOILER ALERT (for those who've not read the books): Death drastically affects Harry as a result of losing his parents, then later in the books we get the deaths of Cedric Diggory, Sirius Black, and Dumbledore (these three are deeply traumatic). That's before I even get to the bloodbath of the final novel (the deaths of Dobby, Fred Weasley, and Snape in particular are profoundly sad).

I don't quite know what you meant by "serious death" but all of those are as serious as it gets. :)