The girl went into the kitchen to do the dishes and had been followed by the woman.
Police alleged the woman had grabbed the girl's face on either side of her mouth and squeezed, and had then punched her in the shoulder.
The woman had denied the squeezing and punching, but admitted a push.
The girl had gone to the lounge and sat down in a chair, where the woman had smacked the girl either once twice across the legs, with a set of keys in her hand, and told her to go to her room.
Judge Walsh said there were no injuries and the assault was at the minor end of the scale and he noted the girl did not want to see the accused convicted.
The judge said the woman "wishes to have the chance to gain employment" and was concerned a conviction would make that less likely.