1st & 2nd level Taxonomy in a block view, ???

i set a taxonomy vocabulary trigered by an argument, i show this taxonomy terms vocabulary list in a block.

now i want to add a 2nd level vocabulary for each term in the 1st level and to show that 2nd level only if the user press on the 1st level term.

Example:
- animals
- cars
- names

and than when the user press one of the above, Example, 'animals' he should get :

-> animals
-dogs
-cats
-monkeys
- cars
- names

Should i create a new taxonomy vocabulary ??? Any ideas ?