Quiet children quarrel: this trick always works!
As a daddy of two little girls I can say that this trick worked every time (to be used by children from about 2-3 years):
When the children play together peacefully, at some point inevitably comes the time when something is quarreled and quarreled: both want, for example. play with the same toy or one has taken something away from the other or or or ... (use any typical reason ...)
Next step is often that mom or dad be consulted: "Dad, now I want to play with it!" etc. pp.
Now do not make the mistake and try to make things fair and to settle ("Now you let them play shortly, then it's your turn" or something similar). Not a good idea. Often goes in the pants.
I always say, "That you have to make out among you!"
Lo and behold, the children do it under their own steam. Often one child gives the toy to the other and the other child puts something else out for it and it goes on peacefully.
Worked so far in 99% of all cases!
My amateur explanation (no idea if that's true): The children always want to pull mom / dad on their side to get the "upper hand". If they realize that this does not work, they agree quickly, so they can continue playing.
What are your tricks?