Results: spanking someone elses child

the other day my sister and her husband were watching my younger children while my wife and i were christmas shopping. my 4 year old knows not to go outside alone to play but she snuck out and did anyways. my brother in law brought her back in and talked to her then spanked her (fully clothed, on the bottom a couple of swats with his hand). i was very upset about this even though i probably wouldve done the same thing and i know he loves her and would never hurt her and also has been married to my sister for almost 20 years. he is a gentle man who never raises his voice in anger or anything like that with my kids or his own. so i know for a fact he did not HURT her, however, as a parent i just feel as if this was not his place to do so and i am very tempted not to take my kids back over there again. what do you think?