I have to admit, I might have slapped SO if he said something like that to either of our kids too. That is NOT okay. Ever. I don't care how stressed he is. I'm not sure what I would do about it but SO would be in DEEP smurf. I guess I would talk to him and ask him what in the smurf he was thinking. Then I would have him talk to DD and apologize to her. He needs to make it right with her more than he needs to make it right with you. Kwim?
Not that any parent want their children to get hurt, things like this happen and I am sure DD did not mean to hurt DS. Don't be offended for what I am about to say but you should not have slap him infront of the kids and far worse DH should not have said what he said to DD. I think when everyone calms down you both need to talk and DH need to apologize to DD for the words he used.