当我回顾童年的时候,我仍然以为那是我一生之中最快乐的时光.
Looking back to my childhood,i still believe that was the happiest time that i spent in my life.
或：
When i look back to my childhood,i still believe that was the happiest time that i spent in my life.
两句的先行词都是 time,在定从中做spent的宾语,that可以换为which也可以省略