Is it possible to detach physics-activated meshes from the parent and let them go their way. See:
AFAIK, it’s not possible to break a compound physics shape. If you want to break connections, you will have to create a constraint and at some point, break that constraint.
PhysicsJoint.DistanceJoint which should be pretty much what you need. Then, set an impulse threshold for breaking.
And apply an impulse to break it.