Taxonomy - parent not related to child?

Hello,

Please, consider the following example:

I create one vocabulary named "Twentieth century".
Inside that vocabulary I create the terms for each decade (e.g.: 60’s, 70’s, 80’s…), and inside each decade I create the sub-terms for each year of that decade.

This vocabulary should look something like this:

Twentieth century (vocabulary name)
(…)
50’s (term)
--1950 (sub-term)
--1951
--1952
--1953
--1959
60’s
--1964
--1966
70’s
--1975
(…)

I will then tag each node using the years (sub-terms) in that vocabulary, I'll allow multiple selection of sub-terms so that each node can span for some years.

For instance, imagine one given event lasted between 1952 and 1955, I'll then create one page named "Event X" and select from the vocabulary list the sub-terms "1952", "1953", "1954" and "1955".

I would them expect to get all the events belonging to a certain decade, by selecting the term... but that's not happening

Now, using drupal’s jargon ;0), why can’t I get all the child sub-terms from one given parent by selecting that parent?

I’m only using the default taxonomy.module, must I have another module installed? Or is there a special tag I must apply to the url?

Thanks, Gonçalo Dumas