It sounds like they already exist in level then user clicks something and actor a is now parented to b?
I think you are making things harder for yourself by having seperate actors already in level then attach dynamically.
Because essentially you need to tell one actor get the actor from level and then add it as a child actor to this actor, which is doable but overly complex for what your after i think.
What i would do is first look at is have your level that allows the customization of each object and when user clicks confirm you send the transform and class of everything to a new actor that generates the required amount of child classes and assign them their respective class and location to this bp. This is the overly simplified explanation of what you need to do, as you are going to need to learn spawning, constructor script, delegates and probably interfaces. This can be all done in blueprints but would transfer to code for performance.
You really should think about what your users are gaining here as, unless you are confident in the techniques i mentioned you are really going to struggle with this as you have a lot of moving peices, so if you are fixated on this then start simple sketch out the logic on paper then delve into how do you get from a to b.
Hope this helps