In 吃完晚饭后，我们去了维多利亚海港看烟花, why is 了 after 去 and not after 看? In my chinese book I saw the following sentence.

6年初二晚上他们一家人去看了烟花 。

That has sense with the following grammar point for verbs in series with 去
9.5   Two verbs: One indicating purpose When a simple sentence has two verbs and the second verb indicates the purpose of the first verb, such as 来 (lái), 去 (qù) and 用 (yòng), the perfective aspect particle 了 follows the second verb if it is a past event. 下午我去超级市场买了一些水果和面包。 Xiàwǔ wǒ qù chāojíshìchǎng mǎi le yìxiē shuǐguō hé miànbāo. I went to the supermarket to buy some fruit and bread in the afternoon. (Incorrect: 下午我去了超级市场买一些水果和面包。 ) 我用手机给我妈妈发了一个短信 , 告诉她我不能回家吃饭。
But later in  book i saw this one.

吃完晚饭后，我们去了维多利亚海港看烟花。

Why is 了 after 去 and not after 看? Is it because is a subordinate sentence?
It's an exception.
With verbs in series, when the object of the first verb is a proper noun, the aspectual 了 is placed after the first verb.
So the construction becomes: &quot;Subj. + V1 + ASP 了 + proper noun + V2&quot;
In your example, you indeed have a proper noun after the first verb:

吃完晚饭后，我们去了维多利亚海港 (Victoria Harbor) 看烟花

Another example:

我去了北京找我的朋友 —— (北京 proper noun: V1 + ASP 了 + V2)
我去图书馆借了一本书 —— (图书馆 common noun: V1 + Obj. + V2 + ASP 了）