This one is so simple--just strips of masking tape and then dark blue watercolor with some salt sprinkled over it. After it dries you take off the tape, paint on the moon with white paint and use watered down blue watercolor to put shadows and lines on the trees.
Here's the original idea at Art Projects for Kids. I like hers much better...almost enough to re-do mine! (My salt did something very different so I didn't add the white snow dots.)