Most of the time there’s at least two solutions in game development 
Honestly, if I see nested child actors, I would probably think about some other solution.
However, answering your question, there’s this node which seems to handle your case (when Include Descendants is ticked)
Get All Child Actors | Unreal Engine Documentation
Also, I’m not sure if it works, but child actors basically spawns an actor and attaches it to a blueprint, so you can try to call GetAttachedActors in Greasing Point as well