remove parent but not constraints

I am making a new game prototype and I have a rigid body enemy with constraints to make a ragdoll. The way it is set up currently is: each piece of the body is the child of a motion cube of sensor type physics (cannot use dynamic because children are rigid). I have the motion cube detect a bullet and send a “die” message. The body pieces get the message and remove their parent. The undesired result is that their constraints get removed too and defeats the purpose of the ragdoll physics. I feel as if I am not taking the correct approach at this. Is there a better way?

I think I might be able to fix this with collision mask and group. (To be able to make the motion box dynamic). Still does not address main issue.