SIOUX FALLS, S.D. (KELO AM) - Police say two twelve-year-old Sioux Falls boys are charged with aggravated assault in an incident involving an eight and nine-year-old boys.     Officer Sam Clemens says all four are neighbors on East 54th Street and were together last Monday night.     Clemens says one twelve-year-old pushed the nione-year-old who slipped on the ice, hit his head and was knocked unconscious.     Clemens says the other twelve-year-old pushed the eight-year-old down and got on top of him.  The eight-year-old manage to wiggle free but the older boy kicked him in the head.     When the nine-year-old woke, the boys went their seperate ways.     Clemens says one of the victim's mothers called police the next day and the twelve-year-old were taken to Volunteers of America.