In parent class make a getter function to return your boom (make sure the function is public!)
parent.h
UFUNCTION()
USpringArmComponent* GetBoom();
parent.cpp
USpringArmComponent* AParent::GetBoom()
{
return CameraBoom;
}
Add a constructor in your child class
child.h
public: // make sure its in public
AChild();
child.cpp
AChild::AChild()
{
GetBoom()->SetupAttachment(GetMesh());
}