unity how to make a object a child of colliding object code example

Example 1: how to reference a child object unity

private void Start()
    {
        parentObject = GameObject.Find("Parent");// The name of the parent object
        childObject = parentObject.transform.GetChild(0).gameObject; // the parent index (starting from 0)
    }

Example 2: unity create a child object

// spawns object
        objToSpawn = new GameObject("Start");

        // add Components
        objToSpawn.AddComponent<Rigidbody>();
        objToSpawn.AddComponent<MeshFilter>();
        objToSpawn.AddComponent<BoxCollider>();
        objToSpawn.AddComponent<MeshRenderer>();

        // sets the obj's parent to the obj that the script is applied to
        objToSpawn.transform.SetParent(this.transform);