Childrens minds seem to conjure up all types of things. My mom used to read to my children at bedtime, after hearing Jack and the Beanstalk and the line fe fi fo fum I smell the blood of an english man etc. my son would not sleep with the lights off or eat toast (because it was made from bones he said) but he would eat a sandwich...go figure. It took him almost two years to get over this. The same story had never bothered his brother at all. Eventually they will either forget or out grow the fear. I am aware it can be trying, just give lots of hugs and reassurances.