Problem with hierarchical custom taxonomy

Hi!
On my site I’ve set up a hierarchical custom taxonomy, currently with 2 levels.
My classification scenario demands I use only one term, preferably one of the children terms (if there are any), otherwise one of the 1st level terms but never simultaneously on a parent + a child term.

My problem is that I’m getting all the Child Term’s posts on the Parent Term’s page. Lets say my Parent Term is Animals. I’m getting this:

Animals
 -->Reptiles
 -->Lion
 -->Mammals
 -->Snake
 -->Dog
 -->Cat
 -->Gecko

Instead of:

Animals
 -->Mammals
 -->Reptiles

Or even this:

Animals
 -->Mammals
    -->Dog
    -->Cat
    -->Lion
 -->Reptiles
    -->Snake
    -->Gecko

I’ve been going over the Codex and the forum looking for a possible solution to my problem but with no avail.
Any idea on how to solve this?

Thanks!