My kids are 6yo and 3yo and tend to be quite physical with each other. I think it is my 3yos way of getting her big brothers attention, and now that they are fairly evenly matched I leave them to sort it out and only intervene if it's getting too rough.
They were on a bouncy castle at the weekend and every time one of them jumped on the other, as they would at home, the other kids were gasping and saying they were 'at it'. These were slightly older kids but I was a bit shocked at how they saw this brother/sister wrestling.
So should I be telling them not to play like this, they don't do it to other kids and are nice to one other, most of the time...