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//with the index
parentGameObject.transform.GetChild(0).gameObject
//or with the name
parentGameObject.transform.GetChild("name").gameObject
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// To acces a child of a gameObject use the combination below
parentGameObject.transform.GetChild(0).gameObject
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//Instantiate Prefab
GameObject originalGameObject = Instantiate(prefab);
//To find `child1` which is the first index(0)
GameObject child2 = originalGameObject.transform.GetChild(0).gameObject;
//To find `child2` which is the second index(1)
GameObject child2 = originalGameObject.transform.GetChild(1).gameObject;
//To find `child3` which is the third index(2)
GameObject child3 = originalGameObject.transform.GetChild(2).gameObject;
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//For unity engine
GameObject.transform.GetChild(The child index).transform;
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GameObject originalGameObject = GameObject.Find("MainObj");
GameObject child = originalGameObject.transform.GetChild(0).gameObject;