The Parent Trap
The Parent Trap is an American film from Walt Disney, directed by David Swift (1961), based upon the 1949 book Lottie and Lisa (Das Doppelte Lottchen) by Erich Kästner.
Identical twins Susan Evers and Sharon McKendirck (both played by Hayley Mills) are unknowingly separated shortly after birth, only to meet again at summer camp. Although initially hostile (due to having identical appearance) they quickly realize that they are sisters. The two initially use their identical appearance to meet the parent that they'd never meet. However when it becomes apparent that their father plans to re-marry, the two scheme to ensure it doesn't happen and to get their parents back together.
The spanking scene
After the girls learn of the marriage plans, Susan reveals the switch to the mother, causing her to fly to California. The mother (played by Maureen O'Hara) decides to head back to Boston with Sharon and calls the two girls downstairs so that she can take Sharon back with her. However both girls appear dressed identically and inform the parents that the one of them that isn't Sharon, who they won't reveal, will not be going to Boston. The Mother scolds them both for getting smart and, grabbing one, tells Sharon to go upstairs and put her suit on, while a couple of smacks on her backside. The girls are however unfazed, with the other one asking the mother if she's sure she's Sharon.
Hayley Mills was born in 1946, so she was 14 when the film was made. She also stars in the 1981 TV mini-series The Flame Trees of Thika.
Maureen O'Hara also stars in a number of other movie spanking scenes.