Hug it out!
If you catch your kids hitting each other, immediately pull them apart and tell them to go to their respective corners to calm down.
Then, face each other and exchange apologies like so:
I’m sorry for (insert wrong doing committed), I did it because (explain why they got upset), and finally,
next time, I will (insert appropriate thing to do).
After the kids have had their say, tell them to hug it out. They may find it awkward at first, but overtime, they start giggling and all is well again.