"Darkness Slipped In" by Ella Burfoot is a gorgeous book to read when your little one is feeling a bit scared about the dark. Darkness is made into a lovely, friendly character who comes to dance with Daisy before bedtime.
I can completely understand why darkness suddenly becomes scarey for young children and how they need to have the light on at night. It's a tricky one because I also know they sleep a lot better when the light is off. I now creep in to their room later on and turn the lights off or shut the door. Soon it will be Summer time and I'll be faced with the opposite problem of there being too much light too early in the morning! But for now, we regularly read Ella Burfoot's book, so that we all remember that there's nothing to be afraid of really and Darkness is actually rather fun.