T1: relationID, meterID, parentID, childID
T2: dataID, meterID, date, amount

        T1                                   T2
-----------------              -------------------------
1  | 1 | null |  2  *           1 | 1 | 01,01,2013 | 100  *
2  | 1 | null |  3  *           2 | 2 | 01,01,2013 | 60   *
3  | 2 |   1  |  4              3 | 3 | 01,01,2013 | 40   *
4  | 2 |   1  |  5              4 | 4 | 01,01,2013 | 35
5  | 3 |   1  |  6              5 | 5 | 01,01,2013 | 25
6  | 3 |   1  |  7              6 | 6 | 01,01,2013 | 15
7  | 4 |   2  | null            7 | 7 | 01,01,2013 | 25
8  | 5 |   2  | null
9  | 6 |   3  | null  
10 | 7 |   3  | null  

DECLARE @T1 TABLE
    (relationID INT, meterID INT, parentID INT, childID INT)
DECLARE @T2 TABLE
    (dataID INT, meterID INT, [date] DATE, amount INT)

INSERT INTO @T1
SELECT 1  , 1 , null ,  2 
UNION ALL SELECT 2  , 1 , null ,  3 
UNION ALL SELECT 3  , 2 ,   1  ,  4 
UNION ALL SELECT 4  , 2 ,   1  ,  5 
UNION ALL SELECT 5  , 3 ,   1  ,  6 
UNION ALL SELECT 6  , 3 ,   1  ,  7 
UNION ALL SELECT 7  , 4 ,   2  , null
UNION ALL SELECT 8  , 5 ,   2  , null
UNION ALL SELECT 9  , 6 ,   3  , null
UNION ALL SELECT 10 , 7 ,   3  , null

INSERT INTO @T2
SELECT 1 , 1 , '2013-01-01' , 100
UNION ALL SELECT 2 , 2 , '2013-01-01' , 60
UNION ALL SELECT 3 , 3 , '2013-01-01' , 40
UNION ALL SELECT 4 , 4 , '2013-01-01' , 35
UNION ALL SELECT 5 , 5 , '2013-01-01' , 25
UNION ALL SELECT 6 , 6 , '2013-01-01' , 15
UNION ALL SELECT 7 , 7 , '2013-01-01' , 25

SELECT a.meterID, b.[date]  
    ,MeterAmt=MAX(b.amount)
    ,SumOfChildren=SUM(c.amount)  
FROM @T1 a
INNER JOIN @T2 b ON a.meterID = b.meterID
LEFT JOIN @T2 c ON a.childID = c.meterID 
GROUP BY a.meterID, b.[date]