Okay I am not able to set the same session for the both notebooks (parent and children).

So my result is to use %run ./notebook_name .

I put all the code to functions and now I can use them.

Example:

# Children notebook
def do_something(param1, param2):
    # some code ...
    return result_value

# Parent notebook
 
# some code ...
 
%run ./children_notebook
 
# some code ...
 
function_result = do_something(value_1, value_2)
 
# some code ...

Thanks to everyone for the answers