How to add force to broken pieces upon impact?

Hello,

I have a whole wood plank (3D mesh) & it's broken pieces. Now I child the broken pieces to the whole wood plank. They are childed is such a way that if you disable the renderer of the parent, it would not make a big difference visually. I then set children's renderer, rigidbody inactive.

Now if I an Object (lets say an NPC like a moving cubical object) do a slightest touch on the parent, I want to disable the parent and set the children active then add the required force on the broken pieces so it looks kinda realistic.

ILLUSTRATIONS :

1. Parent and It's Children

2. On Collision & It's Effect

I understand it has to do with physics and I am not sure as of how to achieve it. All of the above assets have rigidbody as their component & I think I just need to know as of how to apply the force on the children.

I hope I was clear with my doubt.

Thank you for your help and time. Much appreciated.

regards,

Karsnen.