In the season 3 of "game of throne",
I didn't understand that how stark family got Lannister family's two little children as prisoner?
In the 3rd episode of season 3 "Walk of Punishment" it was revealed that Edmure Tully (the uncle of King Robb Stark) disobeyed orders and attacked Gregor Clegane. Edumre won the battle and Gregor fled, but Edmure was able to capture some hostages including Willem and Martyn Lannister. We never see the battle, but it is discussed when King Robb chastises his uncle after the funeral.