Ray Bradbury 
Zero Hour [PDF ebook] 

In this short story first published by Ray Bradbury in the 1951 "Illustrated Man" collection, the game of "Invasion" has been sweeping the country. Children all across the nation pretend to have been enlisted by alien invaders, their job to overthrow their parents, and help their newfound friends take over the Earth. To Mrs. Morris, it’s harmless fun — but to her daughter Mink, it’s far from just a game.