kids driving me crazy!!

Hi, I wondered if anyone has advice to give about how to get my two children playing good together without squabbling all the time. My daughter is 3 and my son 20 months. They can play nice together it just doesn't happen that often and i'm getting so fed up of having to be the referee all the time and not being able to just leave them to play. Has anyone gone through this and found any useful tips that can help turn me back into "nice" mummy instead of "mummy who shouts all the time"!!!??

They are too small to be played alone. It is needed your participation.