SQL> explain plan for select * from dual;

Explained.

SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Plan hash value: 3543395131

--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |     1 |     2 |     2   (0)| 00:00:01 |
|   1 |  TABLE ACCESS FULL| DUAL |     1 |     2 |     2   (0)| 00:00:01 |
--------------------------------------------------------------------------

8 rows selected.

SQL> 

SQL> ed
  1  explain plan for
  2  (    SELECT *
  3         FROM
  4             (
  5            SELECT
  6            v.division_function DIVISION
  7            , v.bu_meaning BUSINESS_UNIT
  8            , v.category CATEGORY
  9            , v.country_meaning COUNTRY
 10            , v.brand_meaning BRAND
 11            , v.proposal_id PROJECT_ID
 12            , v.name TITLE
 13            , v.description DESCRIPTION
 14            , v.parent_project PARENT_PROJECT
 15            , v.project_class_meaning PROJECT_CLASS
 16            , v.benefit_category_m BENEFIT_CATEGORY
 17            , v.feeder_stream FEEDER_STREAM
 18            , v.feeder_stream_detail_meaning FEEDER_STREAM_DETAIL
 19            , v.commodity_meaning COMMODITY
 20            , v.pl_meaning PL_LINE
 21            , v.status STATUS
 22            , v.have_fin_validated HAVE_FIN_VALIDATED
 23            , v.date_savings_start DATE_SAVINGS_START
 24            , v.project_manager PROJECT_MANAGER
 25            , v.location LOCATION
 26            , v.supplier_diversity SUPPLIER_DIVERSITY
 27            , v.pso_indirect PSO_INDIRECT
 28            , v.project_probability PROJECT_PROBABILITY
 29            , round(CLX_CSDB_GET_PARENT_PLAN_ACT3(v.benefit_line_id,null, null, 'NET_SAVINGS', null, null, 64066),0) NET_SAVINGS
 30            , p.short_name PERIOD
 31            , round(decode(p.month, 'YEAR', 0, 'QUARTER', 0, CLX_CSDB_GET_PARENT_PLAN_ACT3(v.benefit_line_id,p.period_id, P.FS_SOURCE, p.month, p.quarter, p.year, 64066)),0) AMOUNT
 32            FROM CLX_CSDB_PROP_V_MAT v
 33            CROSS JOIN CLX_CSDB_RPT_PERIODS p
 34            WHERE p.report_id = 64066
 35            AND instr('ONLY_STRUCTURAL', v.benefit_category, 1) > 0
 36            --ORDER BY v.proposal_id desc
 37            UNION ALL
 38            SELECT
 39            	v.division_function DIVISION
 40             , v.bu_meaning BUSINESS_UNIT
 41             , v.category CATEGORY
 42             , v.country_meaning COUNTRY
 43             , v.brand_meaning BRAND
 44             , v.PARENT_PROJECT_ID PROJECT_ID
 45             , v.name TITLE
 46             , v.description DESCRIPTION
 47             , v.parent_project PARENT_PROJECT
 48             , v.project_class_meaning PROJECT_CLASS
 49             , v.benefit_category_m BENEFIT_CATEGORY
 50             , v.feeder_stream FEEDER_STREAM
 51             , v.feeder_stream_detail_meaning FEEDER_STREAM_DETAIL
 52             , v.commodity_meaning COMMODITY
 53             , v.pl_meaning PL_LINE
 54             , v.status STATUS
 55             , v.have_fin_validated HAVE_FIN_VALIDATED
 56             , v.date_savings_start DATE_SAVINGS_START
 57             , v.project_manager PROJECT_MANAGER
 58             , v.location LOCATION
 59             , v.supplier_diversity SUPPLIER_DIVERSITY
 60             , v.pso_indirect PSO_INDIRECT
 61             , v.project_probability PROJECT_PROBABILITY
 62             , round(CLX_CSDB_GET_PRJ_ACTUALS3(V.CHILD_FS_ID, V.FS_ID, v.benefit_line_id, V.BENEFIT_TYPE, V.BENEFIT_CATEGORY, V.USER_DATA_ID, null, null,  'NET_SAVINGS', null, null, 64066),0) NET_SAVINGS
 63             , p.short_name PERIOD
 64             , round(decode(p.month, 'YEAR', 0, 'QUARTER', 0, CLX_CSDB_GET_PRJ_ACTUALS3(V.CHILD_FS_ID, V.FS_ID, v.benefit_line_id, V.BENEFIT_TYPE, V.BENEFIT_CATEGORY, V.USER_DATA_ID, p.period_id, P.FS_SOURCE,  P.MONTH, p.quarter, p.year, 64066)),0) AMOUNT
 65             FROM CLX_CSDB_PRJ_CHILD_V_MAT v
 66              CROSS JOIN CLX_CSDB_RPT_PERIODS p
 67             WHERE p.report_id = 64066
 68             AND instr('ONLY_STRUCTURAL', v.benefit_category, 1) > 0
 69             ORDER BY v.PARENT_PROJECT_ID desc
 70             )
 71             PIVOT
 72             (SUM(NVL(AMOUNT, 0)) as unadj
 73*            FOR PERIOD IN ('Jul 13','Aug 13','Sep 13','Oct 13','Nov 13','Dec 13','Jan 14','Feb 14','Mar 14','Apr 14','May 14','Jun 14','Q1 14','Q2 14','Q3 14','Q4 14','Total 14','Jul 14','Aug 14','Sep 14','Oct 14','Nov 14','Dec 14','Jan 15','Feb 15','Mar 15','Apr 15','May 15','Jun 15','Q1 15','Q2 15','Q3 15','Q4 15','Total 15','Jul 15','Aug 15','Sep 15','Oct 15','Nov 15','Dec 15','Jan 16','Feb 16','Mar 16','Apr 16','May 16','Jun 16','Q1 16','Q2 16','Q3 16','Q4 16','Total 16','Jul 16','Aug 16','Sep 16','Oct 16','Nov 16','Dec 16','Jan 17','Feb 17','Mar 17','Apr 17','May 17','Jun 17','Q1 17','Q2 17','Q3 17','Q4 17','Total 17')))
 74  /

Explained.

SQL> 
SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
Plan hash value: 2886296404

----------------------------------------------------------------------------------------------------
| Id  | Operation                   | Name                     | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |                          |  1207 |    20M|   951   (1)| 00:00:12 |
|   1 |  VIEW                       |                          |  1207 |    20M|   951   (1)| 00:00:12 |
|   2 |   TRANSPOSE                 |                          |       |       |            |          |
|   3 |    SORT GROUP BY PIVOT      |                          |  1207 |   433K|   951   (1)| 00:00:12 |
|   4 |     VIEW                    |                          |  1207 |   433K|   950   (1)| 00:00:12 |
|   5 |      UNION-ALL              |                          |       |       |            |          |

PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
|   6 |       MERGE JOIN CARTESIAN  |                          |   132 | 70224 |    95   (2)| 00:00:02 |
|*  7 |        TABLE ACCESS FULL    | CLX_CSDB_RPT_PERIODS     |     8 |   328 |    23   (0)| 00:00:0
|   8 |        BUFFER SORT          |                          |    17 |  8347 |    71   (0)| 00:00:01 |
|*  9 |         MAT_VIEW ACCESS FULL| CLX_CSDB_PROP_V_MAT      |    17 |  8347 |     9   (0)| 00:00:01 |
|  10 |       MERGE JOIN CARTESIAN  |                          |  1075 |   438K|   856   (1)| 00:00:11 |
|* 11 |        TABLE ACCESS FULL    | CLX_CSDB_RPT_PERIODS     |     8 |   328 |    23   (0)| 00:00:0
|  12 |        BUFFER SORT          |                          |   135 | 50895 |   832   (1)| 00:00:10 |
|* 13 |         MAT_VIEW ACCESS FULL| CLX_CSDB_PRJ_CHILD_V_MAT |   135 | 50895 |   104   (0)| 00:00:02 |
----------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
---------------------------------------------------

   7 - filter(TO_NUMBER("P"."REPORT_ID")=64066)
   9 - filter(INSTR('ONLY_STRUCTURAL',"V"."BENEFIT_CATEGORY",1)>0)
  11 - filter(TO_NUMBER("P"."REPORT_ID")=64066)
  13 - filter(INSTR('ONLY_STRUCTURAL',"V"."BENEFIT_CATEGORY",1)>0)

28 rows selected.

SQL> 

1. you know what a materialized view is
2. prnt_mat_v is a MVIEW
3. chld_mat_v is also a MVIEW
4. rpt_tbl is a table

FULL TABLE SCAN OF PRNT_MAT_V
FULL TABLE SCAN OF CHLD_MAT_V
UNION-ALL OF THESE TWO SCANS
INDEX LOOKUP OF RPT_TBL
MERGE JOIN CARTESIAN BETWEEN RPT_TBL AND THE UNION-ALL OF THE OTHER TWO TABLES

First collect statistics correctly DBMS_STATS.GATHER_TABLE_STATS(null,'prnt_mat_v').  Check to see if for your version of Oracle, the percentage sampled is 100 by default.  If not, then use the percent parameter to make it so, otherwise accept the default.

Second check to see that you actually got valid stats.  NUM_ROWS on DBA_TABLES and NDV on DBA_TAB_COL_STATISTICS.

Third do the decomposition tests.

a. the query without the pivot
b. the first subquery (no pivot)
c. the second subquery (no pivot)