Key-Value Pair Design

Hi,
I have to log the Details of the incoming xml message into databse.
But the values logged will vary with the message.So I cant fix the mumber of columns.
I thought of using table in which the fields are logged as Key-Value Pairs. The table looks as below.

TransactionID ColumnKey ColumnValue
1111 PONumber 123
1111 Sender xxx
1111 Recever yyy

using dynomic query i was able to get the results as follows
TransactionID PONumber Sender Receiver
1111 123 xxx yyy

Till now every thing was fine. but now i got new requirement where i have to identify each column with its parent. For example if we consider the line items of the PO, table may look like below.

TransactionID ChildKey ChildValue ParantKey ParantValue
1111 PONumber 123 null null
1111 Sender xxx null null
1111 Recever yyy null null

1111 ItemName soap ItemID 123
1111 Quantity 4 ItemID 123
1111 UnitPrice 2.2 ItemID 123
1111 ItemName Brush ItemID 222
1111 Quantity 5 ItemID 222
1111 unitPrice 4.4 ItemID 222

I am unable to design the database which satisfy the requirement of the reporting.
I not even know how to query the data which is logged like this.
Help me by giving the inputs to design databse for the above problem and to query the data .

advance thanks
Srinivasa Mahendrakar