Not 100% sure what you mean, but let me try a deconstruction here:

今天晚上，我和爸爸妈妈一起放花炮。

No 了 really needed here, because a time was specified (this evening). 

爸爸用上打火机点上一根香烟，我拿着烟，吸了几口后就开始放花炮。

There's one 了 here at the end - when describing a series of actions, the 了 is only placed on the last one. 

我点了一个，花炮喷出五颜六色的的火苗，真是太美丽了！

Two 了 here - the first one is describing a past action, the second one, 太美丽了, is part of a standard phrase 太...(usually and adjective or adverb) ... 了。

冲天炮太危险了，

Same deal here, 太...了

我不敢放，爸爸帮我放，我只在旁边看。 不一会儿，花炮放完了，

This 了 is a little different, it expresses a changing state. 

我们就高高兴兴地回家了。

This one expresses a past action - they went home. 

Make sense?