Destroy specific child object of prefab object in c# / Unity - unity3d

Question

I search a lot but couldn't find any answer. My problem: I instantiate several Gameobjects of a Prefab, each consists of two child objects: an obstacle and a scorezone. I want to destroy the scorezone (only of that specific instance), when the player touches the obstacle. So in Runtime my hierarchy looks like this:
-ObjectPrefab(clone)
---Scorezone
---Obstacle
-ObjectPrefab(Clone)
---Scorezone
---Obstacle
...
So I need to find ONE child gameobject (not the Transform) to then destroy it. I tried several codes but none worked. Here is the code with the three alternatives I tried, but none of them worked.:
private void OnCollisionEnter2D(Collision2D collision) {
if (collision.gameObject.tag == "obstacle") {
Alternative 1:
Destroy(GameObject.FindWithTag("scorezone"));
Alternative 1 comes really close, but it destroys all scorezones of all instantiated objects.
Alternative 2:
Destroy(collision.gameObject.transform.parent.gameObject.FindWithTag("scorezone"));
Alternative 2 would sound logic to me but I get the error message "Transform does not contain a definition for "FindWithTag".
Alternative 3:
foreach (Transform child in collision.gameObject.transform)
{
if (child.tag == "scorezone")
{
Destroy(child.gameObject);
}
}
}
Alternative 3 does not give an error but actually does nothing at all when the player hits the obstacle.
I really appreciate some help, thanks in advance!

Answer 1

Store the reference!
If it is really a "prefab" so meaning you have it as an asset and instantiate it on runtime the best way (in my eyes and performance wise) would actually be to use a serialized field:
// Put this component on your obstacle
public class ObjectController : MonoBehaviour
{
// Drag the according object here via the Inspector in the prefab
// => when the prefab is Instantiated this field is already referenced
[SerializeField] private GameObject _scoreZone;
// Read-only access to the reference
public GameObject ScoreZone => _scoreZone;
}
This field reference is stored together with your prefab asset. So every instance of the prefab already has its own child references set up.
and then do
private void OnCollisionEnter2D(Collision2D collision)
{
// Rather use CompareTag which is slightly more efficient
// and adds some security (== silently fails if typo, CompareTag throws an exception instead)
if (collision.gameObject.CompareTag("obstacle"))
{
var controller = collision.gameObject.GetComponent<ObjectController>();
if(controller)
{
Destroy(controller.ScoreZone);
}
}
}
By Index
Alternatively but error prone you could simply use the index. You already know that there are only 2 childs, the first one being the Scorezone with index 0 so you can also do
if (collision.gameObject.CompareTag("obstacle"))
{
var scoreZone = collision.transform.parent.GetChild(0);
Destroy(scoreZone);
}
By Name
Using the name you could also use Find - I would not recommend this! It is error prone and slow
if (collision.gameObject.CompareTag("obstacle"))
{
var scoreZone = collision.transform.parent.Find("ScoreZone");
Destroy(scoreZone);
}