I believe the film you're talking about is the 1959 movie called "The Sad Horse". The young boy is David Ladd, son of Alan Ladd, future husband of Cheyl Ladd. The woman comes to stay at the farm the boy lives at because she is angry with her husband. She had slapped him during an argument in which he slapped her back. The husband comes after her to reconcile, but they continue arguing. She starts to slap him again but he threatens her with a spanking, telling her "it won't be a little slap this time". She unfortunately changes her mind.