She was calling her friends, very loudly, a bunch of bastards.
I said "stop that now, that is not appropriate in front of all these children."
She walked off muttering, I then asked her to apologise, to which she said she wouldn't as she wasn't sorry, but wouldn't do it again. 
I repeated that I would like her to apologise for causing offence, and she again refused saying that she didn't need to apologise to anybody if she didn't want to.
All the other moms were muttering about her being a disgrace, the life guard eventually got her to grudgingly apologise to the room and told me later that they already knew who she was because of her attitude.
My friend who was with me told me she wouldn't have got involved, but I feel that this sort of behaviour needs to be challenged, albeit I normally do it in a more gentle, "Come on,guys" sort of way. I was just so shocked at the word and the volume that I went straight into cross mom mode.
What I'm really asking is AIBU to speak to an unknown child about her behaviour?