How do I reset the location of an empty object with python and NOT move its children?

Question

I would like to use the Python API to:

1. set the parent of a set of objects to an empty object
2. then set the location of the empty object to the midpoint of the bounding box of all objects

but not move all the objects

Step 1 works ok - but I am stuck on the second.

I start with this situation:

and after grouping things look like this:

and

with the origin of e (the empty object) set at [0, 0, 0]

Now if I try to re-center the empty doing this:

def recenter(obj, point):
    try:
        obj.data.transform(mathutils.Matrix.Translation(-point))
    except:
        print('obj has no data')

    print('translation', obj.matrix_world.translation)
    print('location', obj.matrix_world.translation)    

    obj.matrix_world.translation += mathutils.Vector(point)

    print('translation', obj.matrix_world.translation)
    print('location', obj.matrix_world.translation)    

    return(obj)


tidy_names(locals())

recenter(e, [1.5, 1.5, 0.5]) 

I end up with:

Is there a shorthand way of doing this? I suspect I could recurse through all the children and reset, but that seems quite insane...