Learnt from the school teachers: -
1) Don't ask them to 禮讓, kids usually don't understand at young age
2) Tell them to "take turn", the one who is playing the toys can keep playing but need to pass it on to the others who want to play later
3) Persuade the kids to ask instead of just grab the toys from the others' hands
One big and useful tactics is to give them compliments for being nice to the others, e.g. great job, good boy, how nice you are... My kids are 4 and 2-1/2, fighting everyday but they are learning. They are their own best friends and the best enemies.