How to display the childs of the parent with posts?

Hello. I need your help.
How to display the childs categories with they posts of some parent category?

On sidebar i have something like that:

* Parent Category 1
* Parent Category 2
* Parent Category 3

If user click one of these Parent Categories on sidebar he will see:

* Parent Category
        * Child Category 1
                Post 1
                Post 2
                Post 3
        * Child Category 2
                Post 1
                Post 2
        * Child Category 3
                Post 1