Music to scare the crap out of little kids by

Slayer. The beginning of Hell Awaits, or all of Raining Blood.

Some of the soundscapes between tracks from Annihilation off The Wicked (Nile) sound quite frightening / daunting...so are most of their tracks actually.

Mantra by Tool
 
Cowell - The Banshee

first time I heard that is was pretty scary sounding, but after a while you can actually understand whats going on. But yeah for kids, this is great.