这句话最好说成：How happy they are?
一定要用 How happily they ________!一定要填动词,happily是副词,修饰动词.
How happily they are playing!勉强,他们在读书,怎么玩得高兴呢!
How happily they are reading!
不可以说 How happily they are having fun!have fun 就是玩的开心,不能用happily修饰
注意一定要用正在进行时.因为前面的Look.不是啊.是are having.are having 肯定是错的，除非你给的句子有误。have后面必须有宾语，你的句子后面没词了。除非是这样Whata happy time they are having!