It's been a while since I've had to think along the lines of books for kids that young, especially if they're prone to nightmares, but off the top of my head:

Anything by Lloyd Alexander, but especially the Chronicles of Prydain (The Book of Three, etc.).

Cornelia Funke, though I don't know how good the translations are. Note that the Inkheart trilogy may cross the nightmare threshold and IMO isn't really as good as her other stuff. A lot of the rest may be aimed a little young for a bright tween, but at least Dragonrider and Thief Lord should be pitched about right. Igraine the Brave might be too young for her, but could help with the nightmares.

I like what I've read of the Bartimaeus trilogy (Jonathan Stroud), but I don't know much about the books overall or anything else by the author.

Also, the first three HP books ought to be relatively nightmare free.