2 objects, same script, OnTriggerEnter individually

I feel like this question has been answered here multiple times already, yet non of the answers really help me in my particular issue. I have an object, which has has it's own OnTriggerEnter() for different purposes, but this object also has a child object, which has a OnTriggerEnter() as well. The OnTriggerEnter() of the child acts like that: if the child object touches something, then the main objects does something. It works perfectly fine, when there is only one main object with it's child, but when I copy it, only one of them actually works. I figured out it has something to do with the fact, that those are 2 different scripts and, when the main objects accesses the child object, the child object alters the parameters for all main objects. How to make them work individually? Here's a simplified code:

The child:

OnTriggerEnter(Collider other) { if(other.gameObject.tag == "tag") do_Sth_in_parent == true; }

The parent:

if(do_Sth_in_parent) { do stuff }