Took DS to the park today (2yrs 3months). He was playing quite happily with a 'car' climbing frame type thing when 2 other boys (5 or 6 yrs old) jumped on and started saying, 'lets make it shake so he falls off' 
Now, I know they were only 5/6 yrs old but I was quite miffed, not to mention a bit
that they said it in front of me!
DS has had this sort of thing but never reacted/ didn't really understand it as malicious, but today he was clearly upset.
I said to 'all', 'play nicely'
They then followed up with saying 'you're stupid!' to DS

So, I glared
but didn't say anything more and they soon moved on.
How would you handle it?