Bokurano

During a summer camp, 15 children find a grotto by the sea. Deep within they discover electronic equipment, and later the owner, a man called Kokopelli. Kokopelli claimed to be a programmer working on a brand new game, in which a large robot has to defend the Earth against fifteen alien invasions. He persuades the children to test the game and sign a contract. Barely a moment later they mysteriously awaken on the shore believing what happened was just a dream. (Anime News Network)