Bokurano

Bokurano (ぼくらの, Bokura no, lit. "Our") is a Japanese science fiction an anime television series directed by Hiroyuki Morita and produced by Gonzo that aired in 2007. It is based off of a manga series of the same name. It was broadcast in Japan from April 8 to September 25, 2007 SUN-TV and TOKYO-MX, as well as other stations. Episodes were released in Japan on eight DVDs between July 25, 2007 and March 26, 2008.

Plot
In a summer vacation, a group of 15 children discovered a mysterious man living in a seashore cave, along with his high-tech gadgets. The man claimed to be a game developer creating a video game with a giant robot defending Earth from 15 alien invaders, and asked those children to test the game for him. The kids agreed happily at first, but soon they began to find the game horrifying, not only because of its realism but also because the first fatality among them happened right after their first victory. Worse yet, they had no idea how to stop or leave the "game."

Official

 * Link to Wikipedia's page
 * Bokurano anime on My Anime List
 * Bokurano manga on My Anime List