This has happened to me a few times recently. I usually don't say anything but just move my own child out of 'range'. The problem is that then, when a child inevitably gets hurt, I feel guilty for standing by and letting it happen.
The latest incident was in our neighbourhood. All the mums were there and the kids were riding bikes. One boy (age 4) started doing this thing where he would ride really close to someone and stick out his leg (nearest them) but just miss them. He did it to me and my toddler and a few others but then he did it to a child and his foot made contact and they both fell off their bikes. Of course, I felt guilty for not trying to prevent what was obviously going to happen and my guilt made me say to the mum that I thought there was 'some intention' there. She said no, he just wasn't looking where he was going. Then he did it again and she saw so she said, oh he's just trying to show off his new balancing skill. I think it's great that she is sticking up for him and I probably shouldn't have said anything. She did tell him not to do it again though so maybe I prevented another mishap, who knows.
Anyway, what's the best way to deal? Mind your own business? Passive aggressively say loudly to your own child 'lets move away before someone gets hurt'? Ask the parent to stop their child? Try to talk to the child? None of those options seem great to me. Any more ideas?