So the question is; what to do? I've read that parents preferably should stay out of the conflict and let them sort it out themselves, and sometimes I've seen that it works. But sometimes my instincts just kick in, and I have to do something, interfere, explain, comfort, try to guide them, or separate them at worst.
I know I shouldn't worry, that it will pass, it's all in phases, that it's a natural part of their development. But it's still difficult, and strange, and frustrating. It even is a little bit fascinating, because suddenly, in a fight, as if someone swung a magic wand over their heads, they are the best of friends again, kiss, hug, make up, and it's all forgotten. Weirdos!