How do I transform the position of my cube AWAY from the larger cube its attached to?

Hi,

I have a small cube which is parented to the side of a larger cube.

(see below)

However, the small blue cube may end up on any other side of the larger orange cube.

What I'm asking is although I can null the parent the cube is part of, how can I make the cube fly in a straight trajectory AWAY FROM the side of the cube that it is currently sitting on.

FOR EXAMPLE

If the small cube was on the left of the big cube, it would fly in the direction of the arrow:

I apologize if I've not made myself clear, gave it my best shot!

Many thanks,

Farid