DS (6) and DD (4) are still at it. They would squabble about who is breathing in the most air, given half a chance.
Strategies depend on level of patience/tiredness/how many hours it's been going on for...
Ignore (so long as DS isn't being really violent).
Make them play separately.
Offer them 10 minutes of my time each (timed on a kitchen timer), and toss a coin for who gets first go.
Respective cool-down corners.
Warning for nipping/kicking/etc, then the re-offender is removed from the room for 4/6 minutes. I really object to violence, esp. as DD always comes off far worse (though she does wind DS up something rotten...)
Gin and tonic.
Allow them to call each other the three worst things they can think of (they have to think very, very carefully, as they only have three insults!!) Fortunately, the worst insult they know is "poo-poo head". It normally ends up in laughter, but I only bring it out if really desperate!!
Isn't it wearisome? Good luck...