The kids were happily play-fighting in the backyard, giggling as they wrestled.

play-fighting 是由动词 play（玩耍）和动词 fight（打架）组成的复合动词，加上-ing 作为现在分词形式。play-fighting 指"假装打架的玩耍"，是一种模拟打斗的游戏行为。