I love this trailer for The Dark by Lemony Snicket, illustrated by Jon Klassen.
The first time I watched it I was very impressed by the voice talent and then learned at the end that it was Neil Gaiman reading, so that explains that.
Everything works together on this trailer - the reading, the use of children to show the demographic, the fact that it's clearly a beautiful book in terms of both text and illustration, the music, and the brevity.
I've got a board on Pinterest called Trailer Park where I am collecting some of my favourite children's book trailers. Suggestions more than welcome!