Multiple gameobjects have same script, which has a List of instantiated gameobjects

My gameobject “String” have a list named “points”. The list have many gameobjects “point” - they are childs of “String”. When I instantiate multiple gameobjects “String” and try to access one “point” of one this, the system has error: “Argument is out of range”.
How can I effect exactly the “point” of one object “String” on among many instantiated gameobjecs “String”?

It’s probably this:

int j = points.IndexOf (obj);
if (points[j].tag == "point")

You need to check if IndexOf returns -1, which means it’s not in the list.