How to display current manu on wordpress page display parent and sub-menu fo on sidebar

Question

I have a main navigation and all parents have children.

Example:
Parent Page A:

• Child A1
  - Child A11
  - Child A12
  - Child A12
  - Child A13
  
• Child A2
• Child A3

Parent Page B:

• Child B1
  - Child B11
  - Child B12
  - Child B12
  - Child B13
  
• Child B2

If we are click on Child A12, sidebar should looks like:
Parent Page A:

• Child A1
  - Child A11
  - Child A12
  - Child A12
  - Child A13
  
• Child A2
• Child A3

Thanks.

Answer 1

This is a simple CSS tweak! use firebug see the class being added to the child and sub-child menus and accordingly write the css!