Should grown-ups lie to you?

Molly has never been afraid of getting a needle because she knows the Needle Ninja will reward her bravery with a basket of Japanese lollies, cakes and toys. Carl has never heard of this person and thinks Molly has been tricked by her mum and dad. But is there any harm in believing in the Needle Ninja? And is it OK for parents to tell their kids stories if it makes them feel better?