If you define a Blueprint as a ChildActor component the Parent spawns the child instance of that class. It is the purpose of a ChildActor component to spawn a specific Blueprint and sort of glue two Blueprints together in a Parent Child relationship.
It sounds like you are calling a function on an orphaned child.
To keep things simple you should try to keep the communication going in one direction so e.g. from Parent to the child, however the child can talk to the parent with “GetParentActor”.
If you want to glue two specific instances of Actors together at run-time you could use the Blueprint node “AttachToActor”.