I often take the boys out and while I do not hang out with them, I can (almost) always see them when playing.
Generally, like Julz, i leave them to sort out their own little tiffs, unless I see it getting out of hand. If another child knowingly hurts one of mine, I do go down to their level and tell them it is not nice behaviour to hurt someone else - I do it calmly. If it is one of my children that started, I make them apologise, and if the other parent is nearby, I apologise to them as well
Basically I treat all children as I do my own. Okay to be honest, I am actually harsher on my own. But if I see a child crying I will go offer comfort and take their hand to find their mom/ dad. If another child does something that deserves a "well done" I give them one, and if another child misbehaves, i let them know it is not acceptable behaviour