How to assign GameObject to child for serialization

Question

I want to assign each item I create with a GameObject of some sort and then when I create a new object (called Item) it will have the assigned GameObject as a child.

For that I have a class of a scriptable object which holds a public GameObject called "gameObj" within it:

public abstract class ItemObject : ScriptableObject
{
    public int id;
    public GameObject gameObj;
}

Then, in another class I want to have something of this sort:

public class GroundItem : MonoBehaviour, ISerializationCallbackReceiver
{
    public ItemObject item;

    public void OnBeforeSerialize()
    {
        this.gameObject.transform.GetChild(0) = item.gameObj; //WRONG CODE, NEED HELP HERE
    }
}

The purpose is to set the gameObj from the given ItemObject.item as the GameObject for the GroundItem.

The purpose is in the end to have lots of scriptable items of all sorts (like bread, sword, stone etc) and each one will have a GameObject assigned to it, and once I create a new GroundItem game object I will simply assign the scriptable object item and its child will have the game object itself (which includes all the visuals, special scripts etc).

For reference, in the following link the person is doing this from minute 4 to minute 6, but with a sprite instead of a game object.

Anyone knows how it should be written? Is it even possible?