I think you did the right thing.
I wish I had done something when a couple of kids behaved very weirdly with my ds last year.
Girl about 11, brother about 6 - had seen them before and noticed them because the girl was very confrontational with the adults in the park - very odd, never really seen anything like it. And the boy was just a bit wild.
They had tried to get ds - 3 at the time - to play with them but when he did they would tell him to go away, then come back for him again til I asked them just to leave him alone as they were clearly on the wind-up.
Next time they did similar - I kept an eye as they encouraged ds to go to the other side of the play[ark (I let them cos I knew I could follow and I really wanted to know what they were up to - suppose I wanted a definitive reason to tell them to piss off or speak to the mother) Anyway, once they thought they were out of my earshot the girl told the boy to "quickly get the sticks - I'll hit him then you hit him" 
I really wasnt expecting anything as cold and nasty as that - quickly grabbed ds and told them if they so much as looked at any of the kids in the park in the wrong way they'd be in serious trouble. The boy atsrted crying and the girl called me a fucking cunt.
Went to the mother (who was not actually in the playpark but in the park iyswim - she rolled her eyes, said "oh ffs - ALRIGHT... we're going anyway!"
I felt really disturbed by it - felt a bit teary so just went home. And I feel utterly crap that I didnt do something - dont know what exactly but I really should have. I decided after the event that if I saw them again I would talk to the girl and find out which school she went to and perhaps call them but I never saw them again.
So, yes, you did the right thing 