That's not really "parenting". Parenting is when you tell one whole layer to inherit the transform properties of another layer.
What you're referring to can be done in a few different ways.
One way is to not bother with the null object layers at all, and just apply the tracking data directly to the effect control points for the ends of the beam.
Another way is to go ahead and use the null object layers and then use an expression to link the effect control points to the positions of the null object layers.
If you need more detail, let me know.