I would have said something nicely to the kids like "oh, that's not a good idea, someone might get hurt".
I was watching a movie and a couple kids were goofing off on the street and a lady yelled out of the window for the boys to go home because they were misbehaving and the boys looked at her and said "yes, mam". It his me that what is most lacking today is that fact that NO ONE feels comfortable saying something to other children about behaving.
I know that when we were growing up that if we got in trouble at someone elses house not only would those parents discipline us, they would let our parents know and we got heck at home for it too. If someone was at a store or at the park or where ever and doing something they weren't supposed to, someone would stop it. You had to be on the watch for everyone. It really kept kids in line knowing that no matter what you did, your parents were going to find out about it somehow and they most CERTAINLY did NOT take your side of the story for anything.
I can't believe how many parent get mad if someone even looks at their child wrong. Yes, it is your job to discipline your child but you aren't always there and if your child is doing something that you may approve of, but isn't allowed somewhere else, they should be told to stop what ever they are doing. Everyone is so worried about a lawsuit that they just keep to themselves. It is really sad if you think about it. Having a sense of responsibility for everyone in your community makes living there better.