Shanameh, the Persian Epic of Kings, contains a mathematically relevant story: http://en.wikipedia.org/wiki/Wheat_and_chessboard_problem
In it, the inventor of the game of chess is called to meet the king, who offers to let the inventor choose a reward. The inventor knows that he cannot ask for too great a reward, because doing so will anger the king and rob him of any reward. Therefore he frames his request in the following way: he wants one grain of wheat on the first square of the chessboard, two grains of wheat on the second square, four grains on the third, eight grains on the fourth, all the way up to the 64th square.
The king accepts, puzzled that the inventor would ask for so little, and tells his treasurer to figure out the amount of wheat the inventor is due. A week later, he asks the treasurer how much the inventor received. To his surprise, the treasurer has not even finished calculating the amount of wheat, and he tells the king that the entire wealth of the kingdom would not suffice to give the inventor his reward.
The story ends in a variety of ways; in one, the inventor becomes the new king in a rare mathematical proletarian revolution.
Short when we hear it, since we know how fast exponentials grow, but if you couple this with the exercise of trying to calculate how much wheat the inventor should receive then this should be a fun bedtime story/opportunity to learn a little math for a child.
EDIT: There is also a great volume of anecdotes from mathematics history that could serve as bedtime reading, though you'd have to adapt them to a good format for putting children to sleep. A lot of my early interest in mathematics stemmed from such stories; most vividly, I recall the story of how Pythagoras (allegedly) ordered his disciple Hippasos thrown off a boat for proving the irrationality of $\sqrt{2}$.