OnTransformChildrenChanged

OnTransformParentChanged

using System.Collections;
using UnityEngine;

public class Test1 : MonoBehaviour
{
    private GameObject anotherObject;

    private void Awake()
    {
        anotherObject = new GameObject("anotherObject");
        for (int i = 0; i < 10; i++)
        {
            var go = new GameObject($"go {i}");
            go.transform.SetParent(transform);
        }
    }

    private void OnTransformChildrenChanged()
    {
        Debug.Log($"transform chnaged frame={Time.frameCount} ms={Time.timeSinceLevelLoad}", gameObject);
    }

    [ContextMenu(nameof(RemoveStandard))]
    private void RemoveStandard()
    {
        StartCoroutine(DoRemoveStandard());
    }

    private IEnumerator DoRemoveStandard()
    {
        Debug.Log($"deattach standard frame={Time.frameCount} ms={Time.timeSinceLevelLoad}", gameObject);
        transform.GetChild(0).SetParent(anotherObject.transform);
        yield break;
    }

    [ContextMenu(nameof(RemoveFixedUpdate))]
    private void RemoveFixedUpdate()
    {
        StartCoroutine(DoRemoveFixedUpdate());
    }

    private IEnumerator DoRemoveFixedUpdate()
    {
        Debug.Log($"currentFrame = {Time.frameCount}", gameObject);
        yield return new WaitForFixedUpdate();
        Debug.Log($"deattach standard frame={Time.frameCount} ms={Time.timeSinceLevelLoad}", gameObject);
        transform.GetChild(0).SetParent(anotherObject.transform);
    }
}