Go all out; say you're her Fairy Godfather.

"Err...hello Cinderella." You say. "I have come to tell you that you should go to the ball tonight. Regardless of what your sisters tell you, you have every right to be there."

"Wha...what do you mean? How did you know that...? Who are you?" She stutters, not knowing how to respond.

"Cinderella, I am your fairy godfather." You say. You bite your lip, praying to God that she in gullible enough to fall for it. Luckily for you, you see her eyes light up.

"Are you serious?" She asks excitedly, then her expression starts to die back down. "You don't look like a fairy."

"Well...uh...I can't exactly walk around with wings out and everything, can I?" You ask.

"No, I suppose not." Cinderella's face beams once more. "I can't believe it! I always dreamed I had a fairy godparent looking over me! I knew this day would come!"

"Well, it's here." You say, forcing a smile. "You are going to that ball and you're going to woo the prince!" Cinderella squeals happily.

"But...but I don't have any clothes other than these ratty rags." Suddenly her eyes go wide. "OH! You're going to magically make me a beautiful dress, aren't you?"

"Errr...."