Before you send your kids out to the playground, practice with them what it's like if you're standing back and you want to join a game, how you do that. Or ​practice with them how you take turns in a game. Practice with them how you introduce yourself because sometimes they're new to the playground. And they really will learn those things before they get out there.
Also teach your kids how to ask for help, and let them know that if anybody is being pushed, or shoved, or names are being called, those are the times that you remove yourself from the game and then you go and ask an adult for help.
