Lying to children is acceptable when it produces good results.
Case in point:
Right after getting my teeth pulled, I had blood running down my face and my head swelled up like a balloon. Also, I was still a little groggy so I was walking with a lurch. Zombie-like, I shuffled up to the prescription counter to get the prescription for the painkillers filled as I waited for the medicine they had already given me to kick in. There was a little boy waiting with his mother to get something filled, who knows what. He just couldn't stop staring at me, in terror. Finally, I said, "THIS IS WHAT HAPPENS WHEN YOU DON'T BRUSH YOUR TEETH!" and scared the living crap out of him. I might be a monster, but I'll bet that he's going to brush his teeth three times a day for the rest of his childhood.