A group of children playing in the yard 为什么不是A group of chil

are playing是谓语动词,playing是-ing形式,是非谓语动词,有谓语就叫句子,没有谓语就不是句子.A group of children playing in the yard只是一个短语,肯定有其他的动词作谓语部分.

A group of children are playing in the yard是正解

A group of children are playing in the yard “有一群孩子在院子里玩耍”，这是一个完整的句子。
A group of children playing in the yard “在院子里玩耍的一群孩子”，这不是一个完整的句子。