Translation:The little boys have kept quiet for an hour.
It depends upon how you look at the sentence. If "have kept quiet" is considered an "event" that happened at a fixed point in time in the past → passé composé. If the "have kept quiet" is a process or a continued state of being → present.
It is very common to use the present tense in French with depuis, whereas in English you'd use the present perfect. However, this is a sentence where either present or passé composé will work.