The wrench characterizes the forces and torques acting on the respective linkages. Part of this is motor torque, the other forces and torques are loading the mechanical structure (and motors down the chain).
If you have followed the Denavit Hartenberg convention in defining the coordinate systems and you have a rotational joint the joint torque is the z component of the torque part either the parent of the child wrench (depending on the motor orientation).
If you did not follow the DH convention, you should locate the axis which is co-located with the motor axis and use the torque component from the wrence corresponding to the co-located axis. If you do not have such an axis, you need to define a new coordinate system which has such an axis and make the coordinate system transformations form the parend/child coordinate system to the new one. Afterwards transform the wrench to the new coordinate system atteched to the motor and select the torque component corresponding to the co-located axis.
Please make sure the regard any gearboxes which the motor might have when calculating any actual motor torques.