twinkle in (one's) eye

1. A facial expression that denotes happiness, amusement, or knowledge of a secret. Just when Rita thought all hope was lost, Jared got a twinkle in his eye that told her he had a plan to get them both out of that sticky situation.

2. A hope or plan that is not yet in existence. You were not even a twinkle in your mother's eye when that photo was taken.