Got it. So the only way to "pass a parameter" with %run is to define it as a variable in the parent notebook and use that variable in the child notebook. This works because both notebooks are executed in the same session so the variable my_var is available in both notebooks.

Parent Notebook:

my_var = "this is a parameter I want to pass" 
 
%run ./my_child_notebook

Child Notebook:

print(my_var) 
 
>> "this is a parameter I want to pass"