UE5.3 Reorder BP_ThirdPersonCharacter components

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());
}

result