How to select rows from a hierarchical query with the lowest level

Assuming kgs_id is the primary key of both tables (which is a little strange), join the tables before doing the connect by:

select kgs_code, kgs_name
from (select * from dic_kgs join dic_kgs_strings using(kgs_id))
start with kgs_parent_id is null
connect by kgs_parent_id = prior kgs_id; 

alternatively look up kgs_name after the connect by (this approach can be helpful in more complicated situations such as if not all records in dic_kgs have a corresponding record in dic_kgs_strings):

select kgs_code, ( select kgs_name 
                   from dic_kgs_strings 
                   where kgs_id=kgs_code.kgs_id ) as kgs_name
from dic_kgs
start with kgs_parent_id is null
connect by kgs_parent_id = prior kgs_id; 

Okay, so here's the data model I came up with.

PRODUCT -- to store product information and maintain parent-child hierarchy

id  NUMBER  "Primary Key Not Null"                  
level_code  VARCHAR2    Not Null                    
name    VARCHAR2    Not Null                    
description VARCHAR2                        
parent_id   NUMBER  Foreign Key references PRODUCT(id)                  

ORDERS -- to store orders for products

id  NUMBER  "Primary Key Not Null"                  
prod_id     NUMBER  "Foreign Key references PRODUCT(id) Not Null"                   
order_type  VARCHAR2    "Not Null Default 'Default'"
order_qty   NUMBER  Not Null
order_date  NUMBER  Foreign Key references DATE_INFO(date_key)
delivery_date   NUMBER  "Foreign Key references DATE_INFO(date_key)
Check delivery_date >= order_date"

FORECAST -- to store forecast value for products (store value for higher levels, store value for lower levels after disaggregation from a parent)

id  NUMBER  "Primary Key Not Null"
product_id  NUMBER  "Foreign Key references PRODUCT(id) Not Null"
forecast_value  NUMBER  Not Null
week    NUMBER  "Foreign Key references DATE_INFO(date_key) Not Null"                   

DISAGGREGATION_RULES -- to store which method was used for disaggregating a value from a higher level to lower level and how much percentage got distributed to lower level

id  NUMBER  "Primary Key Not Null"
parent_product_id   NUMBER  "Foreign Key id references PRODUCT(id) Not Null"
child_product_id    NUMBER  "Foreign Key id references PRODUCT(id) Not Null"
method  VARCHAR2    Not Null                    
from_week   NUMBER  "Foreign Key references DATE_INFO(date_key) Not Null"
to_week NUMBER  "Foreign Key references DATE_INFO(date_key) Not Null Check end_week >= start_week"
percent_distribution    NUMBER  Not Null                    

DATE_INFO -- date dimension, has information about start date (has to be Saturday) and end date corresponding to the week in which a particular date falls

date_key    NUMBER  "Primary Key
Not Null"                   
full_date   DATE    Not Null                    
week_begin_date DATE    Not Null                    
week_end_date   DATE    Not Null

As for the bucket number .. I am calculating week start date (date on Saturday, in my case) with the following function

CREATE OR REPLACE FUNCTION get_week_start_date(v_bucket_num IN NUMBER)
  RETURN DATE
IS
  week_start_date DATE;
BEGIN
  SELECT (TRUNC(SYSDATE+2, 'IW')-2) + ((v_bucket_num-1) * 7)
  INTO week_start_date FROM dual;
  RETURN week_start_date;
END;