The story of "the boy who cried wolf" also worked very well for my daughter.
My daughter has told me some very convincing and detailed stories about what she did in her daycare that turned out to be just her imaginations. I was getting worried about this, so I told her the story of "the boy who cried wolf". She actually enjoyed the story so much. After hearing it one more time, she could retell the story herself, even with most of the extra details I made up. She also became very careful not to lie afterwards.
Every child is different. But maybe you can give this story a try and ask your child why no one helped the boy who cried wolf in the end.
