Catherine Ann Downey hit her adoptive granddaughter so hard she broke her arm.
Another time, Downey headbutted the 14-year-old girl. Her brother, also 14, was assaulted by Downey while in the shower.
Yesterday, Downey, 65, pleaded guilty to five charges of common assault - four against the girl the other against the boy.
The girl said the time Downey headbutted her, as she argued with her father, was the worst assault.
She couldn't see properly for a short time after the attack and suffered a bruise to her forehead, police prosecutor Sergeant Drew Morrison told Whanganui District Court.