Typing this really quickly but try this initially:
For your physics asset go into constraint mode and just delete all of the constraints. You want too keep all of your boxes/bones but remove the constraints between them. This way if a bone is told to simulate it will just fall away.
In your blueprint there is a function you can call on a skeletalMeshComponent called something like “Wake all bodies below”. For the parent just plug in the hit bone from the hit result. Your mesh will have to be simulating physics at this point as well which is just a variable on the actor.
I am away from my comp right now but can try to get more exact examples later.