No plot twists, no language, no adult themes or jokes or phrases that would go over the head of a child. Write a story that a child might have imagined, and tell it as a child would tell it. Not to say that you should abandon all your writing skills, but that this story should be straightforward, simple, and good--with nothing to read in between the lines and with a resolution that would leave children rejoicing. In 1,500 words or less, be creative and uncomplicated. Take delight in everything that is good and beautiful, and if you end up fighting bad guys of any sort, make sure you win!