So, I solved it, thanks to AnswerHub 
So here it goes:
Lets say you have a BP “LightConeBP”, which has a cone mesh and a stationary Spotlight.
And you want to add it to another BP actor and communicate to the BP, then you have to define an interface:
In another BP, calles “PreFabBP” you add it as a child actor:
In the events graph, you can call any function that the child implements via interface (see first screenshot).
(Here is the part that has thrown me a bit off yesterday)
This will compile, but not work: (nothing happens)
From the variable, a “GetChildActor” needs to be pulled and then connected.
This works:
If you hook that up to the construction script, you have editor real time changes…
So, you can add BPs to other BPs and configure/drive them.
Which is basically all you want from a prefab?
What else do you need?