I am kind of with you on this one in a sense OP. I think it's quite funny how fully grown adults are mocking you - makes them sound like children more, don't you think? ;)
Anyway yes I wouldn't want my kids or anyones kids to have to hear that kind of thing in the school premises. Of course they will hear swearing but does it have to be from a parent? They are supposed to be the ones teaching children appropriate behaviour. Now I know no adult is completely grown up and nobody can act appropriate the whole of the time, especially when stressed. But couldn't she have done it after she dropped the kids off? Only 5 minutes would've made a whole lot of difference.
I personally though would have called her out on it. Saying "Excuse me. Excuse me. Can you please stop using that kind of language, there are a lot of children around."
If she says it's none of your business, state that your childs welfare is your business and that she should take it outside the school gates.
How are teachers supposed to tell kids not to swear when they hear it from some other kids mother or father on the playground? They might as well not do it if that is the case. Just like you wouldn't get a parent telling a kid to spark up a cigarette, or go out late at night on their own - the same with foul language should apply.
My friend was picking up her children and I came along with her and one of the parents had the most foulest of mouths. Not only to others, but to her child. I also had this experience when doing work experience whilst in school. I believe one of the parents called their child a little C&$%. I don't think this is acceptable at all. Means to me a majority of parents aren't fit to be parents. Because if they are willing to swear and emotionally blackmail their kids, what else are they doing? I know we are all only human but seriously? If you have a child, you make a sacrifice and a pledge to be the best possible person you can be for them.
Sorry this was long, just wanted to get my opinion across. I do tend to write essays. :)
Oh and if you know which child she was the parent of you could possibly talk to her childs teacher and not tell her to tell her off or anything, just to tell her that she was swearing and you are sorry if your child swears, but you aren't the one who has been teaching them that kind of language. If s(he) chooses to take it up with the parent, that would be up to the teachers discretion.