Image you have a grouped objects in the hierarchy, Game object 1 has child game objects 1_0, 1_1 and 1_2 as following:

So how do we get the child game objects 1_0, 1_1 or 1_2 from game object 1? Suppose we have a script attached to game object 1 - GameObject1.cs:
private GameObject go0;
private GameObject go1;
private GameObject go2;
void Start() {
// get game object 0
go0 = gameObject.transform.GetChild(0).gameObject;
// get game object 1
go1 = gameObject.transform.GetChild(1).gameObject;
// get game object 2
go2 = gameObject.transform.GetChild(2).gameObject;
}