In Bettina Büttner’s exquisitely lucid documentary Kinder (Kids), childhood dysfunction, loneliness, and pent-up emotion run wild at an all-boys group home in southern Germany. The children interned here include ten-year-olds Marvin and Tommy. Marvin, fiddling with a mini plastic Lego sword, explains matter-of-factly to the camera, “This is a knife. You use it to cut stomachs o...