The traffic rule says young children under the age of four and to weigh less than 40 pounds must be in a child safety seat.
weigh应改为weighing
应改为weighing。本题考查非谓语动词。不定式作后置定语表示将来。根据句意可知，此处应用现在分词作后置定语修饰children，表示主动和进行。
交通法规规定四岁以下、体重不超过40磅的小孩必须坐儿童安全座椅。