Bokurano - Ours 001 (2010)

Saving the world is hard. Saving yourself is even harder. One summer, 15 kids innocently wander into a nearby seaside cave. There they meet a strange man who invites them to play an exciting new video game. Sounds like fun, right? This game, he explains, pits a lone giant robot against a horde of alien invaders. All they have to do is sign a simple little contract. The game stops being fun when the kids find out the true purpose of their deadly pact.