ChocolatePeanut,
I've been known to swear in front of my children - if someone tackled me about that I would have a hard time containing myself. My children are taught consideration to others, good manners (please, thank you) and tbh I think it is no big deal if they hear me utter sh*t sometimes. That is quite a different scenario to physical abuse.
Back to the situation described - it sounds pretty horrible. This wasn't just a one off smack...I would have to have seen it first hand though to let you know what my reaction would have been.
Having said that, I sometimes wonder if I will appear on mumsnet eg:
"I saw a mum smack her 3 year old daughter's bottom, and then just turn around and walk away and leave her crying whilst shouting "I've had enough of you!!!!"
(yep - that was me last week when dd was being really trying and we were in danger of missing our ferry back to Southampton...I waited for her just around the corner keeping an eye on her btw. I don't generally smack and feel guilty when I occasionally lapse, but she had been pressing all the buttons for quite some time. As it happens, although not pleased that I smacked her bottom, it did seem to bring things to her head and following the aftermath of kiss and make up she was great)
Or,
"I was out for a walk and saw a small child - not much older than one, lying on the floor and crying piteously / desparately whilst him mum stood some distance from him with a stern look on her face and her hands on her hips apparently not caring two hoots. Why didn't she pick him up and cuddle him - he was clearly distressed!
(yep, that was me yesterday. Ds who is 15 months old absolutely will not tolerate the pushchair and wants to be carried all the time and everywhere (he can walk very competently) - that was me trying to teach him that I won't always carry him just on a whim and he needs to either walk or use the buggy - I did give in and pick him up a minute or so later and the elaborate, heart wrenching crying ceased in a nano second)