Them
(2006)

Clementine (Olivia Bonamy) and Lucas (Michael Cohen) finally flee the house, after Clementine appears to kill one of the attackers by pushing him off a roof. The couple flee into the forest but are separated when Lucas can't climb over a fence due to his injured leg. Clementine is captured by the attackers after she finds her car on a dirt road and tries to flee. Lucas finds her abandoned car and beats an attacker unconscious.

Lucas finds an abandoned tunnel system and hears Clementine scream from inside. He stabs one attacker and then flees with Clementine, who follows a younger child says he'll lead them to safety. He leads them to a ladder but stamps on Lucas' hands, making him fall to the ground where he's seized upon by the group. Clementine spares him and manages to make it to the tunnel exit. But it's block, and despite the fact it's beside a busy road embankment, no one sees her and she is taken by the group.

A postscript at the end of the film says that the bodies of Clementine and Lucas were found 5 days after their ordeal. The attackers were all aged between 10 and 15. One of them gave the reason for killing the couple as "They wouldn't play with us". 

Thanks Alasdair!