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You can stream Bokurano (2007) on Crunchyroll with a subscription.

During a summer camp, 15 children, 8 males and 7 females, find a grotto by the sea. Deep within they discover working computers and some 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. All but one of them signs; barely a moment later they mysteriously awaken on the shore believing what happened was just a dream.

Each child must pilot a massive machine to protect Earth, but every victory costs them their own life. This brutal survival game forces young pilots to weigh the fate of the world against their own fleeting futures.

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