I deal with kids directly. I taught high school, and even before I had kids, I was used to dealing with kids -even teens- in public. I always approach them in a positive way, and give them the benefit of the doubt as far as motivation goes. For instance, if a bunch of teens are swearing near us, I would smile, catch someone's eye, and say something like, "Hey, you guys look like you're having a lot of fun. But you probably didn't notice there are little kids around. Could you please watch your language and keep it G-rated while the kids are in earshot? Thanks a lot!"
If there was immediate danger, as described in the OP, I would be more sharp, with a firm "stop!" and putting my body in front of the victim. I might stop a hand from hitting me or someone else, but otherwise wouldn't touch the hitter. If the child was really out of control, I would probably approach a parent, but try to do it in a disarming, sympathetic way. "Hi. Your ds seems to be having trouble controling his body right now, and he's hurting others. I think he's overwhelmed with all the activity, maybe he could use some quiet time away from the playground?"
But, this is tricky. It really depends on judging the parent's demeanor. Again, I always try to project empathy... because otherwise the parent will feel defensive and either 1) lambaste you for interfering (which doesn't bother me all that much) or 2) launch into the kid and way over-react. I find the second scenario to be the most common, and it's why I usually deal with the kids directly, and try to teach ds to handle his own disagreements.