Son in law discipline

I am not suggesting that OP should report SiL to SSs, but in one specific case in my life, making a similar threat, made someone step back and consider their behaviour and address the cause of their problem. Mainly because the person knew me quite well, and couldn't be entirely sure I would not fulfill my threat if the behaviour continued

If you have a good relationship with your daughter, I think you should mention this incident and tell her that it worried you. Then see what she says.

If your SIL repeats this behaviour in your home, send the children to play somewhere and tell your SIL that you do not find this a good way of disciplining children.

He will probably call you names, but I hope your saying something leads to him asking you what you would do in the same situation. So have a good answer ready.