I don't know where your beliefs lie on this, but I'll let you know how we handled it, in case it's helpful.
We taught the kids to pray for Jesus to protect them and get rid of the scary things, and it seemed to help the night-time fears. We also found as they went through 'nightmare stages' that if we said a bedtime prayer with them for sweet dreams that they slept well, but if we forgot one night then they'd have nightmares. There seems to be something in it.

Kerrie