The children gathered on the stage suddenly burst into song.

burst 是动词，意思是"突然爆发、突然开始"。 into 是介词，表示"进入某种状态"。 burst into song 是一个固定搭配，意思是"突然开始唱歌"，形容毫无预兆地开始唱歌的状态。