At the park this morning there were a group of 4, very boisterous boys aged about 8. This play area is specifically for under 8s by the way and the parents were in no way supervising their DC. They were sat on a bench surrounded by luggage which led me to believe they were just waiting for a train from a nearby v. big train station.
I was there will a 2 year old (I'm a nanny) and there were a few other toddlers/babies in the play area. He ran to go and play on the climbing, adventure frame thing and the other boys soon decided that was better than the see-saw they were trying to break. They started to hiss in his face and call him names- he was totally oblivious to what they were doing and was concentrating on walking across a step bridge.
I didn't want to intervene immediately because he was ignoring them, so I was watching at a distance of about 5m.
Within a few seconds they then progressed to pretending to kick his face (stopping their foot less than 10cm from his face) at which point I ran over.
I wanted to say "how would you feel if somebody bigger than you tried to kick you" and it came out in French as "do you want someone bigger than you to kick you in the face?" 
I told them he was only little and they should be careful if they are going to play in a park for smaller children. I said I would tell their mums if they bothered him again. I went over to mention it to the mums anyway, and they just shrugged and said boys will be boys!
What would of been the best course of action in the situation? Fortunately my charge didn't seem at all fazed, and if they had kicked him I don't know what I would have done.
Sorry it's long, didn't want to drip feed.