Please help me lol movie i saw in 2011

[AirThatis]

A movie i watched somewhere in 2011 , a kid entered a door and he was taken to a different parallel universe where he was captured by some kids his age that lived in a jungle in a small castle thing , they later became his friends and the real enemies were the government who caught those kids and put them in a machine and turned them into grown men to fight for the government . ( sort of like in the avengers where captain America was transformed into a muscular man only they were transformed into grown men from children ) The government also had an arena with sand really reminds u of gladiator , where they put people there like the children , and they had to fight this huge metal circle ball robot with tentacles . Later in the movie the protagonist ( the kid ) finds the control room of the robot and destroys their city or something . In the end he returns to the real world and cries because nobody believes him what happened and his mom was sick worried and the kid returns to school.